Is it a bones animation? Without knowing, I'm just going to assume it is since that is what my suggestion is based on. Since vector animations should update the vector position relative to model position and world position, that should work fine. With bones animation I believe the vertex is thinking it's in the same actual position but the bone is fooling it to move relative to the bones not the zero point of the model. This is, of course, just a guess. It should be easy to check though, just make a small model using vertex animation and test that with vec_for_vertex. If this is what is happening with you then I guess it could be a bug with the new system that you might have been the first to catch.

hopefully that'll help.

*edit. oops I didn't read the post properly I'm guessing "ertex" means vertex animation. In that case, take what I said about bones and vertex animation and maybe it only shows the correct numbers one frame 1 with no anim.