Hi... I'm trying to do this:

have a 3 sec overlay video, that wants to repeat endlessly over the main video

On my "old" video editing program, it was a right-click over the video and "allow to extend overtime".

Haven't been able to do this on VPX (i have searched long in forums and google)

This same task can be used for the other way round: Have a short video that user wants to use endlessly for background

Thanks!