Well, first off, I can tell you why I switched. It was mostly due to me actually wanting to be an editor as an actual career, and Premiere is much more used than Vegas professionally (and my Video production teacher laughed when I told him I used Vegas. lol) However, that doesn't mean Vegas is a bad program, by any means. Of course you could make very solid videos just using Vegas (as I'd like to say I did), but it just isn't used professionally.
Anyways, why I like it more has to do mostly with that it can do everything Vegas can do (and more), but easier. For example, it was a huge pain in the a** to make something rotate well in Vegas, but in Premiere I literally can just set a dial to how much I want something to spin in a certain time frame. As far as I can tell, Premiere seems to have more effects than Vegas, and it can edit audio more efficiently and so much easier.
Another neat thing about Premiere is that you can have multiple sequences. Basically, they're different timelines that you can edit multiple things in at the same time. In my case, I always edit gameplay-type crap in one sequence, and my live-action stuff in another sequence, to help avoid clutter.
Hopefully that makes sense, if not, I can pull out screenshots to help. lol