Since the Proto Slavs merged in OTL with the Vandals and Vistula Veneti to create the OTL Slavs and became powerful in Central Europe and Eastern Europe, is it possible for the Proto-Balts to merge with the Vandals and Vistula-Veneti with the balts (Balts gain Vandalic and Veneti characteristics like the OTL Slavs) instead making the Balts more powerful and numerous and the Slavs marginalized?
I will repeat what I have writen before (Vistula) Veneti, Slavs and Antes are three geographicaly separate names for people speaking what we today call Slavic languages, yet they belonged to three more or less distinct material cultures. What can be confusing is that people speaking the Slavic langages that would be recorded in history as Wends/Winden/Veneti inherited that name from an earlier group living in roughly the same area but they themselves never used that name for themselves. It was mostly their germanic neighbours that called the slavic speaking people who settled in those parts by the name of an earlier people. The ancient period Vistual Veneti were quite probably some kind of a celtic ofshot though some try to connect them with the proto-balts which adds another layer of confusion since Baltic and Slavic languages would not completely separate until around 500 AD so if some people in 1st century AD can be considered proto-balts based on their culture than they are also in a way proto-slavs.
What I guess you want is to have Balts migrate to the west and that could have considerable butterflies in very near future from a necesary PoD. The subject is hotly debated but as research continues in eastern parts of central europe it is becoming more clear that one of the driving forces that separated future Balts from future Slavs were the germanic tribes, especialy the Goths and their migration from the Baltic to the Pont. The picture that emerges from the late 4th and early 6th century is that those speakers of Balto-slavic languages that were part of the Chernyakhov culture (a mix of germanic, slavic and iranic speakers from which emerged the historic Ostrogoths and Visigoths) went on to become what we call today Slavs and moved to fill the gaps left by the germanic rush to the west, those that were outside that culture and most identified as members of Kiev culture went to partially become what we today know as Balts. But even then things are not quite as clearcut as it might seam since both Kiev and Chernyakhov evolved into and influenced the Penkovka and Prague-Korchakov cultures that are the first cultures conected to the historic Slavs/Veneti/Antes.
In addition to the Balto-Slavs of Kiev culture that went to become both Balts and Slavs, you had a culture whose name currently I can't remember but had some kind of pots in its name, that was mostly responisble with the emergence of historic Balts though it also influenced the creation of Prague-Korchakov culture (but had no influence on Penkovka as far as it is currently known) and was located in today's Latvia, Lithuania and northern Belarus.
The problem for the people that would go on and become what we call Balts is that unlike the people that became what we call Slavs is they were not part of an organised territory that could be called a state which Gothic ruling familly provided to the would be Slavs, therefore giving them a higher level of organisation than their northern neighbours.
Historicaly speaking it is very hard to screw Slavs and wank Balts due to the geographical issues where they lived and the fact as far as it is possible to follow material cultures that more or less can be brought into conection with would be Balto-Slavs the southern section from which the Slavs would emerge were allways more numerous(at least based on the current level of excavations).