Hi
From past experience if there is no envelope the unused space between spaced apart events appears to be rendered as a black frame which is composited with the other tracks.
If you fully mute a track by clicking the mute button vegas does not render the muted track. So bearing this in mind my question still stands. Added to it. Is applying a mute envelope the same as clicking the mute button. If so I would assume that Vegas does not render muted segments of the track. I can find no mention in the Vegas user manual.
cheers
N