I'm not familiar with Vegas Pro, but busses on a recording console function kind of like your master stereo fader. All channels on the mixer are automatically assigned to the master fader (unless you mute them).
You can assign one or more channels to a buss and then control those channel's volume with the buss fader. The buss then acts as another volume control for whatever channels are fed into it.
You can also assign one channel to more than one buss. A good reason to do this is to get a stereo mix of multiple channels (ie. a drum kit mix) so that you can control the volume more easily when you mix down. If the drums are too loud at mixdown, you can then just grab the two busses that you assigned the drum mix to, instead of trying to turn 8 faders down equally at the same time (which, correct me if I'm wrong ... is pretty much impossible on a computer).
Hope this makes sense and hope it helps.
: )
