Hook up audio, select in prefs, arm track/tracks for recording. If you need to loop, just create a selection on the timeline you want to loop, and enable playback loop. Playback the loop section as it loops around it will create multiple takes and you can right click and select whichever take you want.
If you're doing ADR the talent will likely need to see what they're talking to.
Drop original clip into Vegas, add pips, anything will do. This helps the talent get into rhythm.
Add an audio track and arm it for record.
The rest as per above.
An external monitor for the talent to watch helps get the mic and them away from your noisy PC. Using the busses and mixer in Vegas you can provide the talent with a headphone mix that can help them a lot as well. ADR is not an easy thing for talent to do.
Wow, it is that easy wih Vegas? I was expecting some kind long instruction/tutorial. But seems that this is not necesary. I found an old Spot posting but can't seem to get to