Some video files have a delay set for their audio, and this is saved to the preferences of the file. You can check it with softwares like Mediainfo, here is a file report like that attached as a photo.
How does Vegas import these files? They keep the delay when you add the video onto the timeline, and you need to adjust the audio sync manually, moving the audio event backwards ot forward, or it automatically corrigates the delay and places the audio event on the timeline in sync with the video?
I think Vegas Pro ignores MP4 audio offset/delay metadata. Not sure, though.
Mp4 audio is delayed 40-90 ms because it is not interleaved like avi.
Only ones I've had problems with were in vfr video.
Trying to get it perfect is futile -- mp4 audio often drifts within about 10-15 me, and as soon as you get it right on one sw or hw player, it is farther off sync on another.
The files concerned are the MTS files from my FullHD camcorders. So maybe better leaving the audio track untouched when importing? I am observing the playback carefully, but cannot decide if the imported video is in sync with the audio or not. Sometimes it seems as if there was a bit of delay, sometimes as if it was not there. I want to keep the project in proper sync.
Never judge latency by the Vegas preview. It's not a player.