Boris & After Effects do similar things, but they are basically special effects & text animation tools. Boris actually just came out with Red for Vegas, and I've heard a lot of great things about After Effects Pro, which just came out. The best way to think about After Effects is that it's PhotoShop for video.
Both are very expensive and have quite a learning curve, from what I've been told (I don't own either one). If you want to do things like remove the sky from shots, create moving masks, have much better control over chroma key settings, etc., then it would be worth it to at least check them out. I don't have the time for it, so I just outsource that kind of stuff.
After Effects is a stand alone app so you don't really use it "in" VV. You can frame serve out of VV to AE however so that is sort of using it "in" VV. I have used AE for a while and once you get the hang of it it is extremely easy to use. There is a learning curve with any sort of compositing program. (see below)
>>>Are they some kind of plug ins?<<<
See above for AE portion. Boris Red is a plug-in that will soon be available for VV. I used it for a very very brief time and I just did not like the interface or how slow it ran - but that was pre Windows XP and hyper threading and gigs of Ram and so on so I would think a lot has changed over the last few years. You can decide for yourself by going to the website at http://www.borisfx.com/products/RED/
>>>Do vegas really need them?<<<
Some will argue this but - yes! VV does need them if a person really wants to dig deeper into title design, effects work, compositing and the like.
>>>As a video editor, do i really need them?<<<
No. And the important word is "editor" - they are not editing programs. If all you do is edit than they will get in your way. If you like to play, have the time to play, and have the money to blow - than go for it.