Hi Louis,
You didn’t mention the spec of your computer or your microphone, but it should be possible to record directly to the Vegas timeline without problems. One thing to watch out for is positioning the mic as far from the PC as possible to avoid HD noise.
The fact that you have succeeded with one recording means you must be doing most things right. I would do it in smaller stages – maybe 2-3 minutes at a time. Make sure the drive you’re recording your audio onto (File / Properties / Audio / Recorded Files Folder) has plenty of space and is reasonably well defragmented.
Don’t get too hung up on being in perfect synch – record it roughly where you want the commentary, then “S” to split and you can move your audio to exactly the right position afterwards.
Keep us in touch with how you’re going.
Peter Wright
Perth, Western Oz
http://www.allroundvision.com.au