vMix Live Production Software Pro - Manual (Page 89)

vMix User Guide
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5. A negative audio delay should also be used to compensate for the delay of routing all audio externally.
This can be done from the
Recording Setup window and also Settings -> External Output for the Stream
You may need to adjust this to suit your particular setup, but in general a delay of -80 for 30p and -100 for
25p works best.