You can record an unlimited number of audio tracks in Vegas, all one at a time while listening to the tracks that are already done. The key is to use Vegas to record and not sforge.

1. Put your first track in Vegas.
2. go to the insert menu and choose audio track. This puts a new audo track on your timeline.
3. Push the track record button to arm the track. (round button)
4. Push the record button on the transport (next to the loop, rewind, play button, etc)
5. Record your track. You should be able to hear your prerecorded track as well as the track your recording.
6. Add as many tracks as you want, then mix them all using the individual track volumes.

Vegas is a wonderful multi track recording system, and about the simplest I have ever seen. Your options are nearly endless. However, there is a learning curve and your going to have to use your help menu and I would suggest getting a manual if you don't have one.