My biggest problem with Vegas is that nomatter how good your footage and computer are, the preview just can't seem to keep up with frame by frame changes. Even if it looks perfect, when you export it there's lag some place or another and looks awful. When I do mask in Vegas, I take a screen shot of each frame, put them in the timeline and cut them down, then mask them each individually (mask the first one, copy-paste-adjust). It's the only way I can get it accurate
Plus, like Radical said. You can't really do much with your mask in Vegas. Although you can add a quick glow or drop shadow by using track-motion. I also think that it's easier and more straight-forward on AE as long as you have a basic understanding of the software.