Vegas will do what the file header tells it to do. If the 10fps file is really 10fps then Vegas will keep it in sync. However, if the file header says 10fps but the file is actually 10.023781fps, then Vegas will do what it's told and play it at 10fps, which would be slightly too slow. Generally though, the headers are very close to correct, and most clips are short enough that the difference won't be noticeable.
If the frame rate is varying then you have a nightmare.
Almost no NLE has the ability to handle this, they'd need to read metadata so they'd know what the dynamic frame rate should be. About the only camera that does this is the Varicam and I've had more than one frantic phone call about how to handle it in post.
Even so the Varicam I believe doesn't vary the frame rate, it shoots at something like 90fps and the 'speed' control writes metadata to log how it should be played back.