Sony Vegas is for doing the more basic stuff, like cutting the video and adding music (especially adding music) but can also be used for adding effects and other editing. Adobe After effects is for advanced effects or creating custom intros. I usually create the video in Vegas, then run it through After Effects to add complex effects and pizzazz, before I upload it or do whatever. I would not try to use After effects until you know how to use Sony Vegas. I find it to be many times more complex, and would not bring very much benefit unless you are trying to add lots of effects, or use very advanced effects, both of which I would not recommend if you are new to video editing.