The last time I used Vegas, I discovered that it had a very narrow view of what constituted a 'valid' 29.97fps file. I was working with AVIs at the time, which store their framerate as a vulgar fraction, and it turned out that Vegas would only handle a rate of *exactly* 29.97 correctly. Anything else, such as 30000/1001 (which is more accurate) resulted in Vegas repeating or dropping frames.

I reported this to Sony, who claimed that there was no problem. I gave up in disgust and switched to a professional NLE.

I didn't try anything at 23.976, since I was working on a video project at the time, but frankly I wouldn't trust Vegas with non-integer framerates any further than I could comfortably spit out a rat.