Given Placing 5.1 Positioner in Center -- feeds L and C and R:

1) How do you get a 2-channel camera audio clip to "sound" only in Center, plus LFE?

2) How do you get a 1-channel camera audio clip (EITHER Left or Right channel) to "sound" only in Center, plus LFE?

Elementary, dear Steve. You simply click on the appropriate speaker in the panner to (de)active it. The LFE has a separate option at the top right.

Thank you!

Vegas looks like a great way to ADD 5.1 sound to .m2t files just before burning an HD DVD. But, it seems you have to buy the AC3 codec for $200 if you want to export an AC3 file.

Liquid has a similar limitation, so you make a DVD with 5.1 and then find the AC3 file and xtract it. I'll try this.

...it seems you have to buy the AC3 codec for $200 if you want to export an AC3 file...
Most people buy Vegas bundled with DVD Architect, which includes a licensed Dolby AC-3 encoder accessable to Vegas. Total street price somewhere between $400-500 US, there've been some upgrade prices from Vegas to Vegas+DVD as well.

I have DVD Architect. I wonder why the "buy the encoder" message popped-up?

Have you registered DVD Architect yet?
That's usually the reason why.