Elaborating on your question by providing a specific example should get you more answers. AFAIK nothing has changed in this regard with Vegas since day one. On the other hand I can think of ways in which Vegas has supported Variable Frame Rates since day one.
I just started using ShadowPlay to capture my desktop, and when I bring the resulting mp4 files into Vegas the audio drifts out of sync with the video (while playing it in Media Player has no issues). So I am too assume that the Variable Frame Rate is the problem.
What I have done that works is to transcode it with MeGUI (FFMS Index and AVISynth) to a constant frame video, then bring that into Vegas (in which there are no sync issues).