Put your 16×9 footage on a 16×9 project. Put your 4×3 on the same timeline.
Now, rigt click on the little square (pan/crop) on the 4×3 clip. When the window opens on the top, click on the (untitled) when window opens select 16×9 widescreen. This will put black on the top and bottom of your 4×3. You will loose some footage on the top and bottom, but usually it’s OK even if you did not plan for cutting off the top and bottom.
JK
1. Capture your DV as you did.
2. Create a new audio track in Vegas.
3. Input your guitar into the Mike input of your sound board.
4. “Arm” the record botton (on the left of the audio track header).
5. Press record and watch and listen (on a Headset) to your DV as you play your guitar. It will record on the sound track of Vegas. You can repeat it as you need to.
6. Match the resulting sound track to your DV picture and sound.
In Vegas Pro8 and Vegas7, when you go: File->Capture Video->HDVorSDI
when the capture window comes up for HDV, there are boxes for timecode In & Out.
I do not know about Vegas 6.
JK
What I did was I made a separate Vegas project for each scene (4-5 minutes each). Then imported each scene into a Master Project. There were about 20-25 scenes.
In the Master Project the nested (scene) projects handle as a clip, you can shoten them etc. But most importantly each sub (nested) project can be reopened and edited separetly and when closed all the changes show up in the Master Project.