Creating a new entity out of Lithuania sounds a little bit like Vytautas, maybe, but I think you are downplaying the importance of Lithuania Propria somewhat.
Of course, I do: otherwise we'd just have OTL.
But, OTOH, if Vitold is getting a considerably lager state and, in addition, resources of the "Golden" (just using the traditional European name to minimize the confusion) Horde, then Lithuania is seriously marginalized in the terms of territory and population.
Sure, it was by no means a massive or an overwhelmingly most populous part of the country, but because of the extreme decentralization of the Lithuanian state (as you said, each constituent duchy had its own laws and stuff like that), its importance was disproportionately higher because it belonged directly to the Grand Duke, rather than a vassal duke or a viceroy. It committed much more troops and taxes to the state per capita (in the Battle of Grunwald, ethnic Lithuanian banners composed nearly half of the Grand Duchy's army, despite Lithuania Propria only having 20-25 percent of the population).
Now, you touched a critical issue which I intentionally did not mention in the 1st post (more or less expecting it to come up if there is a meaningful discussion). Lithuania of Vitold was more or less a loose confederation which included the lands of Lithuania proper (which also was not exactly a centralized state), "Russian" princedoms of the modern Belarussia and Ukraine and the lands which were just recognizing Vitold as their feudal superior (like Novgorod). Furthermore, the more or less significant Russian princedoms also had been "structured" having the senior prince (direct vassal of Vitold) ruling the main city of a region and his subordinate princes ruling the smaller cities. Plus, each of these personages had a recognized rule to go (with or without his princedom) to a foreign service (a number of these personages, later killed at Worskla, had been earlier fighting for Prince of Moscow against Emir Mamai).
OTOH, approximately at the same time the Great Princes of Moscow started consolidation of their territories into the centralized (by the standards of time) state. Vitold's grandson was a rather mediocre ruler but his son, Ivan III, "The Great", was quite successful in accomplishing this task. So, if (with a little bit of luck, helped by few murders) we have succession of Vitold's line through Sophia, then the end product is a major centralized state in the XV century. Of course, as I said, this "centralization" is still reasonably far away from the modern standards but at least it involves a very strong power of a supreme ruler.
As such, any wannabe Grand Dukes absolutely had to pay attention to the interests of the nobility and governors of Lithuania Propria, and having an Orthodox Muscovite take control would stand directly against their interests. Barring a much earlier centralization of Lithuania, and much wider than what Vytautas did during his reign, I can't see Vasily taking the throne without at least a large civil war.
This is more than a little bit anachronistic: serious confrontation started later when only Catholic nobility got the same rights as the Polish nobility. If there is an equality of the faiths, then the reasons for confrontation are absent.
Creating a new entity out of Lithuania sounds a little bit like Vytautas, maybe, but I think you are downplaying the importance of Lithuania Propria somewhat.
Of course, I do: otherwise we'd just have OTL.
But, OTOH, if Vitold is getting a considerably lager state and, in addition, resources of the "Golden" (just using the traditional European name to minimize the confusion) Horde, then Lithuania is seriously marginalized in the terms of territory and population.
Sure, it was by no means a massive or an overwhelmingly most populous part of the country, but because of the extreme decentralization of the Lithuanian state (as you said, each constituent duchy had its own laws and stuff like that), its importance was disproportionately higher because it belonged directly to the Grand Duke, rather than a vassal duke or a viceroy. It committed much more troops and taxes to the state per capita (in the Battle of Grunwald, ethnic Lithuanian banners composed nearly half of the Grand Duchy's army, despite Lithuania Propria only having 20-25 percent of the population).
Now, you touched a critical issue which I intentionally did not mention in the 1st post (more or less expecting it to come up if there is a meaningful discussion). Lithuania of Vitold was more or less a loose confederation which included the lands of Lithuania proper (which also was not exactly a centralized state), "Russian" princedoms of the modern Belarussia and Ukraine and the lands which were just recognizing Vitold as their feudal superior (like Novgorod). Furthermore, the more or less significant Russian princedoms also had been "structured" having the senior prince (direct vassal of Vitold) ruling the main city of a region and his subordinate princes ruling the smaller cities. Plus, each of these personages had a recognized rule to go (with or without his princedom) to a foreign service (a number of these personages, later killed at Worskla, had been earlier fighting for Prince of Moscow against Emir Mamai).
OTOH, approximately at the same time the Great Princes of Moscow started consolidation of their territories into the centralized (by the standards of time) state. Vitold's grandson was a rather mediocre ruler but his son, Ivan III, "The Great", was quite successful in accomplishing this task. So, if (with a little bit of luck, helped by few murders) we have succession of Vitold's line through Sophia, then the end product is a major centralized state in the XV century. Of course, as I said, this "centralization" is still reasonably far away from the modern standards but at least it involves a very strong power of a supreme ruler.
Being baptised after already taking control is different from having an Orthodox noble inherit the country from the start. The only OTL example of an Orthodox person inheriting the throne of Lithuania was Vaišvilkas, who ruled for a total of two years in the 1260s before leaving, and Shvarn, who ended up killed within a year and possibly never stepped foot in Lithuania at all.
IIRC, both of them had been ruling a predominantly pagan Lithuania so the attitudes and balance of the power really quite different.
It is generally agreed nowadays that Vytautas never actually wished to give Samogitia away with the Treaty of Salynas
I found that the modern historians (and not only those from Lithuania, so don't get it as an offense) made a real headway in the area of a mind reading and applying the modern realities (including national patriotism) retroactively.
Of course, most probably he did not want to give away anything but there is a big difference between personal wishes and necessities of the state. The only thing he had to do differently in this ATL is to stop enticing and supporting the rebels in Samogitia (the rebellions were almost guaranteed, anyway) as a result of recognizing different priorities.
- all he needed was to acquire Teutonic help in his 1399 attack on the Golden Horde as well as make sure they do not attack him while he's campaigning there (and they didn't - they instead attacked immediately after he failed), and then get it back. He recognized the danger of letting the Knights have an immediate border with Lithuania Propria as well as to connect their holdings in Prussia and Livonia - especially since not long after acquiring Samogitia, the Knights began to attack his capital directly, doing so as early as 1402, a mere four years after the Treaty of Salynas.
While the Teutonic Order was not exactly an assembly of the saints, their attitude toward Vitold was to a great degree defined by his attitude toward them. In OTL, the area was important to Vitold because his grandiose plan was crushed at Worskla. In ATL he is successful and all considerations above became insignificant. If he is staying clearly away from the affairs of Samogitia AND is much more powerful than before Worskla, the Order has no reason to blame him for a rebellion and even less reason to attack him in his lands out of a fear of retaliation. And if Vitold is actively siding with them against Poland, the reason for attacking him is gone completely.
And sure, if Lithuania shifted eastward after his death, that would justify caring a bit less about Samogitia - but that's a whole 30 years between Salynas and Vytautas's death during which Vytautas could simply retake that territory. After all, he was as opportunistic as a man could get.
But there is no reason for him to do this (retake Samogitia) because the area is of a zero economic value and, with the established control over Novgorod, loses strategic interest as well. Not sure why would Vitold want an access to the Baltic coast but he gets it in ATL and he also has an access to the Black Sea so the Greater Lithuania is stretching "from sea to shining sea" (sorry
As for messing with the Order, the main problem with such a confrontation remains: even the Greater Lithuania is in a good position for an open confrontation with the Order on a battlefield: it does not have a heavy cavalry, which means that it can afford only raiding type of a warfare and this makes it vulnerable to the Order's attack on its own lands. Not to mention that these huge territories do not directly map into the huge armies. At Grunwald, Lithuanian contingent was significantly smaller than Polish (even if Lithuanian territory was much greater) and much "lighter". Of course, from time to time there were victories in the battles but the risk still was very high. Anyway, with the "move Eastward", immediate confrontation could be delayed until the odds are much better and the risk is lower.
The problem, though, is that Jogaila's killing of Kęstutis and near killing of Vytautas was an exception rather than the rule
Well, Mindaugas and his sons also had been killed by Daumantas and Treniota in 1263.