Routing in Vegas Video 3.0 is done in the tracks themselves and in the busses. As for multi channel routing there must be a multiport audio card and Vegas must recognize it as such to be able to route to and from discreet outputs/inputs.

OUTPUT ROUTING
If you want to route an audio track to a bus you must add a buss to the mixer 1st. As soon as there is more than the Master bus the track header area will have a square button next to the arm for record button. You can use this button for routing the tracks to buses. The busses themslves have this button and it is used for the routing of the bus to the out port of a sound card. Again, if there is not a recognized multiport(multy channel) card on the system then the button will have no effect as it will only route to the default audio out of the PC.

INPUT ROUTING
First you have to arm the tracks. Once armed the button to the left of the arm track button is used for input routing. If you are using a multi chanel card and vegas has recognized it as such then the button will have a number on it correspondiong to the input that is being used for that track. Pressing this button will allow you to select the input source for your track. If there is only a 2 channel card on the system then the button will appear as two little speakers facing away from each other.