As a Romanian, I've been curious as how Vlachs and Magyars managed to remain culturally/linguistical distinct from its slavic neighbours. Did they never get conquered, or how come all of their surrounding neighbours are Slavs but they managed to retain their identity? I am also interested in how vlachs/romanians managed to keep their roman heritage despite being separated from the empire for so long.
To answer for the Hungarians specifically, it may have something to do with the fact that the Magyars came into Europe and settled these regions in the 8th to 9th Century AD, long after the Slavs. After their conquest of the region we called Hungary, they remained a powerful entity in the region.
slavs arrived to eastern europe in the migration period. shortly after they expanded southward, in to the territory of the weakend Byzantine Empire, and later some of them settled on the balkans. when the (pre)magyars traditionally arrived from the eastern steppes and conquested the Carpathian Basin, the southern slav population stuck on the balkans, they assimilated or were "cut" from their eastern european colleagues.
so as u/navinho stated magyars became a strong western nation in less than hundred years, and Kingdom of Hungary succesfully defended its eastern and western borders for the next centuries, and it became a great cultural center of the 14-15th c Europe.
after that Hungary was conquered many times, you can feel the effect of the Mongol Invasion, or the Ottoman and Austrian effect on its culture, or even the effects of the last liberation. the strange, strong framed finnougric language takes away but gives little ground for foreign languages, and the "lawful western nation on chaotic eastern basis" would not blend-in with any neighbouring culture. this and the magyar cultural self-awarness was just too much for the wanna be conquerors of the past millena.
it is also important to note that the Hellenistic/Imperial policy of pacification by (cultural) assimilation, has not been practised throughout the feudalistic era. yes, christian missionarism was enforced fervently, and with it the legitimzation and establishment of medieval law of the land. but rarely more than that. in many (up until modern Nationalism became a palpable force around 1750CE, practically all) instances the ruling class had not only been socially separated from the souzereign but exhibited only marginal, if at all, ethnical or linguistical connectivity to the common people, which themselves constituted no homogenous mass in that respect.
the various nationalistic movements of the last two centuries account for the establishment of the national states and economies that we know today, all that being trademark of the modern era, all of them being th result of armed conflicts and problem-heavy with respect to minorities in every case (at least at one point in time). medieval society accounts for why there was no Eastern Roman Orthodox or Slavonic Orthodox or Catholic or Teutonic Order (or Islamic or Chinese - for a change) people homogenous in culture and customs to begin with.