Do a search for "Audio Track Controls" in Vegas' help.

When you scroll down, under "What do you want to do?" there is a topic titled:

"Assign a track to a bus".

Here is what it tells you: (Pictures are in the help.)

The bus button in a track header chooses the track's primary output. Assigning tracks to busses is especially useful for creating submixes that allow you to adjust the levels of multiple tracks at once or apply an effect to multiple tracks.

You can also access the track routing options from the Mixer Console view. (View | Mixing Console). Again, the help file will explain the details, as does the Vegas user's manual, which can be download from this web site.

The online help and the Vegas user manual are your friend.

Peter