Despite centuries of effort, travel back in time is still not possible ;-)

A stream arrives (v+a) an audio "delay" will retard that presentation of the audio relative to the video.

A negative audio delay is not possible.

A video delay would retard the presentation of the video relative to the audio. Not exactly sure if/how that might be done in vMix.

As has been mentioned elsewhere hereabouts, many times, RTSP/RTMP are not really suitable for production purposes. Most especially when you have concerns about latency. Which is sounds like you have.

I would suggest that's a quirk in the user interface. You can't do anything to a signal that hasn't arrived yet.

A negative delay is not possible without time travel.

The following thread describes adjusting a network buffer in order to achieve a long delay of an output stream.

https://forums.vmix.com/...aspx?g=posts&t=11251

I'm not sure if or how you could delay an input stream.

I'm always trying eliminate latency. I've noticed that some video processing, like layering, scaling, etc. can add a little delay. Sometimes as much as a few frames.

For example, I can set a webcam (Logitech Brio) for 4K but bring it into a vMix project running at 720p30. The webcam stream will be delayed by the requirement to scale it down.