To be fair, the map is rather anachronistic; at the original inception the VGSO still contained Bukovina in Siebenbuergen and the Polish state of Krakow. It was really the aftermath and the eventual peaceful separation of Krakow, whom elected to secede and join the nascent Polish nation, that really solidified the promises offered by the new federation. They would not be forced to remain.
Granted, aside from Bukovina's separation, the separatists movements have not seen much other success. The aftermath of the Great European War was the cause of much destruction throughout the continent; as one of the few bastions of stability, the VGSO was an attractive place to remain for decades afterwards. By the time that peace and prosperity returned, those movements had lost a lot of their support and haven't been able to muster more than 20% of the votes, even in Hungary.
Aside from nitpicking, Midnight has about the right of it. The VGSO was unstable before and after its transition to the new Federation; it was lucky it was able to sit out of the first few months of the war. If it had to throw its forces into the trenches without taking the lessons of the first summer campaigns... morale would have been low enough to threaten the federation. An unstable monarchy would probably fare alright for a few years, and would have to be propped up by its ally. And a war will happen somewhere in the fifteen years after the founding of the VGSO. Perhaps earlier or later, but it will happen.