It sounds like you're creating a feedback loop.

The only reason vMix would be showing your computer's audio levels is if you're also sending your computer audio through vMix.

If you're sending vMix to your computer's output & your computer's output to vMix via the soundboard (or any other means), that's a definite way to cause the problem you're describing.

You don't ever want to route audio back on itself. That's not just vMix--it's everything where you're working with audio.

If you elaborate on what, specifically, your setup is & what, specifically, you're trying to do, I (or someone else) can help you do it without creating feedback.