Lots of different things happened to conquered people, but most that were conquered by the Romans (Gauls, Spanish tribes, ect) were fully Romanized. A big exception to this are the Greeks, who maintained their culture to a large degree, and, as Horace said, in being captured conquered their captors (spreading Greek culture to the Romans).
Its hard to say what happened to the Dacians. Many Romanian nationalists claim their people are directly descended from the Romano-Dacians, but archeology doesn't really back this up.
As for the dark age "barbarian" tribes, both factors you mentioned are accurate. Groups like the Vandals almost certainly existed during EBs time (the Vandals were likely the people known as the Lugii), but more as loose confederations dedicated to the cults of a specific god. What hapened was, in the later Roman period, there were a lot of developments in Germania, including improved agriculture, hence greater populations. Also tribes joined together into more tightly allied groups for mutual defense or offense (usually against the Romans), so more people identified as members of these groups. The population pressures (among other factors) led to migrations, and the larger, more centralized confederations had some military power, so they were able to invade the crumbling Roman Empire. There are also some indications that as these groups entered the Roman empire, slaves and under-class Romans adopted Barbarian culture, so they too would have led to the growth of these groups.
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