The loop is something I am creating. I have lots of experience creating spot-on loops with Acid. The big difference I see between Acid and Vegas is that Vegas is more Frame-based than Audio sample based. Video has only 29.97 samples per second, while audio has 44,100 samples per second.

Here is a blurb from the Vegas Manual:

"Edits that do not occur on frame boundaries can change the appearance of your video. For example if you split two events and move them together to create a cut, splits that are not at frame boundaries can produce a short dissolve in your rendered video. "

Not sure if this is the same type of issue I am experiencing.

But like I said, if I render a 1 measure loop (Fits 1 measure exactly), then drop that newly rendered clip into the project, the loop is slightly longer than 1 measure!

My theory on this issue is that video is 29.97 samples per second, but audio is 44,100 samples per second. So the problem occurs when the beats and measures do not fall exactly on a video frame.

Example:

1) Set project to beats and measures at 175 BPM
2) Turn off quantize to frames
3) Create an A/V loop that is exactly 1 measure long
4) Choose Tools->Render to new track... (use default settings)

Now notice that the new track is 1 frame longer than a measure. This can be verified by turning on Quantize to frames and noticing that the new loop ends exactly on a frame, but the original loop does not end on a frame.

So, knowing that frame alignment is important, it seems that one would need to find BPMs that align exactly with frame boudaries. Though I don't know if this is possible.

Although I know zip about music I think you've created a problem where one doesn't exist. In the case that you're looking at film isn't that different to video except there are only 24 frames per second. Now clearly music cannot fit into 24 beats per second. But this isn't a problem. The music goes along its merry way at 175 BPM and being sampled at perhaps 48,000 times per second and the film still runs at 24fps.
So you can turn off Quantize to frames while you do your thing with the music, adding in as many measures as you desire or hacking it about to your pleasure. Once you've finished and your piece by then will have maybe 10000s of beats you can render the whole thing out and at the very end it MUST finish on a frame boundary so yes it maybe .99999 frames longer than you thought it would be but that's not going to be even vaguely noticable, by then your composition has finished.

What you CANNOT do is edit video in beats and measures just the same as you cannot compose music in frames. That's why you can either cut the vision (either film or video) and then match the music to it or compose the music and cut the vision to it.

Hope I've explained that clearly enough.

Yes, this is what I was afraid of. I guess I will just have to live with 1 extra frame at the end of my loops.

But I have found that if I change my BPM to 180 and set my FPS to 30, everything lines up precicely :)!

P.S. The reason I am so concerne about time and precision is that these A/V loops will be used in a MIDI synced environment.

Hm,
I think if you're trying to sequence both the vision and audio in a loop then you've really got a problem. NTSC isn't even exactly 30 fps!
Normally one would do the music to the vision and if necessary make very small adjustments to the vision if needed to bring it back into sync or to stop it drifiting. You can see that the drift only becomes noticeable after a few minutes and adding / removing one frame of vision to 'slip' the vision isn't noticeable whereas lossing one beat sure is.