If you are going to use after effects cs4 too, then use premiere. Otherwise, feel free to choose whichever. I personally prefer premiere because of the nesting and the track matte, two things that aren't available in vegas. About the mask, in premiere it's not as easy as in vegas to do, but you can still do it internally if you wish. However, if you are going to use after effects too, you will be able to use dynamic links between the two programs, which make editing a lot faster if you would have to often switch between premiere and after effects. In vegas you would have to always export and import with a lossless codec if you wanted to interact with ae, which makes it harder and also more space consuming. Another thing is that the amount of effects and plug-ins available for premiere (and after effects too) overcome those available for vegas.
Another thing to keep in mind is price though: you can find vegas pro for about 100 dollars, whereas premiere is about 1000 dollars, I think.
As for soundbooth vs acid, I don't really know. I own soundbooth, but for what I need to do with the audio (read: cut down the songs), I find myself doing it directly with premiere. I must say that when it came to having to use an actual audio software, I liked
audacity a lot: it's open source and easy to use. If the audio editing you need to do isn't anything too complex, it might be a good alternative.