I can tell you for sure that this is something weird with YOUR system. I've never seen this in 7 years of using Vegas.

What i have seen is that the time left can hit zero while the render isn't complete yet. That's understandable since there's no way for Vegas to accurately know how much longer the render will take. However, if the render is ending while there is still "time left", the time left will always drop to zero as the render finishes.

Then again, if the render finishes, who cares?