After the main mix I usually "save-as" to create a back-up, then mute the dialog tracks or pan music-left & S/FX-right. Same for a spllt-mix which is often requested.
For me.. this is faster than assinging to submasters.
As for the music levels, I usualy ask the producer if they want the original music volume changes or not... some do, some don't.
Vegas does not support multichannel DV audio (just stereo 1/2).
You might want to lay off a second DVCAM mix-minus master (just mute your voice tracks and print to a new tape). This will be better quality- 2 ch DVCAM is 16/48 but 4 ch DVCAM is 12/32...a crummy mastering resolution.
Vegas 7 does support multichannel print for SDI at either 16 or 24 bit (if supported by deck).