Vistulans gould have been a unifying factor instead of the Polans in the area of what is today Poland, Belarus and eastern Ukraine.
As far as any hypotetical Chrobatian/White Croat state surviving the Middle Ages is concerned a researcher first has to define which Croat group is the basis for such a state or even if several of them are.
In the 9th-10th century there were Croat/White Croat/Chrobatian groups (tribes or even tribal coalitions since the OTL Croat origin myth indicates Croats were just the leading tribe of a horde/coalitions that was made out of Slavic and Turkic and even maybe Iranian groups) in eastern regions of the Prague Basin (Between the Czechs and the Moravians) centred around Libice, then there were in the area between Krakow and Nitra (some suggest that Vistulans and White Croats/Chrobatians are one and the same but the proof is circumstantial), then there were well attested Croats in Galicia which were organised as a tribal confederation (claim made by Ukrainian archeologists) centred on Stilsko area, then there were Croats that took part in the founding of Kiev (once again Ukrainian claim), then there were Croats in the vally of river Saale west of the Sorbs, then there were Croats in southern Austria as part of the slavic Carantania (thier influence on the founding of the state is heavly suspected since their settlements were in the political centre of the land), of course there were Croats on the east adriatic coast (but not only confined to the Croat dutchy/principality) and there were Croats in Atica, Argolid and Crete. The ones in Austria, Croatia and Greece certainly are the result of the slavic migrations but as far as we know at this stage of research we have no idea from which of the groups north of the Danube/Carpathians have they originated from. To be completely frank various "southern" Croats might have originated from different groups of "northern" Croats. Than again all the various groups of Croats in Europe (apparently there is an obscure tribe in Afghanistan calling themselves Croats as well, but to day I have failed to find any phisical reference of those guys apart from travels raport of a Yugoslav traveller there in the '60) could have a single point of origin and have been in some way related (beyond the generic slavic link), yet at the same time they could have been completely unrelated slavs, even some of these Croats need not have been slavs. Confusing, yes?
With moderate butteflies in the 9th century instead of Poland, Czechia, Ukraine or Slovenia of OTL we could be now in the 21st century be speaking of Vistulan, Bohemian, Alpine, Carpathian and Dalmatian Croatia. So the survival of various Croat groups through the middle ages is possible (some 160 000 people imigrating from southern Poland to the USA in the last quarter of the 19th and first decade of the 20th century declared their ethnicity as Belochrobatian aka White Croatian ) will they form functioning states is an another matter entierly.
I leave the OP to digest what I have writen as I just got inspiration for a map