Besweet will separate the AC3 track into 5 files. The nice thing is, you can link all the audio and video tracks together, so that when a cut is made, it's shared at the same spot amongst all the files, and they can be dragged and dropped together.

If you are using music from a CD, Vegas keeps both tracks together (you'll see Left and Right side by side in the same track). There are, of course, ways to separate the tracks if so desired. If you are trying to create a 5.1 track from a stereo CD track, that's a whole 'nother discussion. For now, you are probably better off just creating an AC3 2.0 track, which you can do straight from the timeline.