I'm trying to setup an audio delay in negative, since the video delay isn't really smooth, the problem is that when I do this:
It doesn't change anything for the Recording, it works when in positive but not in negative

vMix adds artificial delay to Audio inputs to combat video capture delay so video and audio would be in sync. You can use negative delay on the Audio inputs. It is not per say negative so it would arrive before the signal arrives (requires time travel) but removing that artificial delay is possible using the slider OP has captured, hence the negative delay.