Basically, yes, vector objects remain vector objects, but some attributes will be "flattened" or expanded, for example gradients will be split into a bunch of single filled objects, Multi line text objects will be split into single point type objects, Flash's Transparency will not be recognized etc.
As Flash Layers will usually be converted to single groups in one Illustrator Layer, just use AI's Release to Layers command to restore the original structure.
If you have Flash, do your own experiments to discover other export qualities or limitations.