Are you talking about playing your midi sequences live into vMix ? Do you need a live control on the midi playout ?

If no : just record your midi sequences as wav files and import them in to vMix.

If yes : there are many solutions depending on what midi software/synthesizers you use and how your audio device(s) is/are setup in Windows.

Basically the point to set the audio OUT from your midi to an audio interface that plays the sound. Then simply add this audio interface into vMix as an audio input.

It might get a little complex if you only have a single audio interface to do it all. As the case may be, just use a Virtual Cable to patch things directly.

THere's been a lot of talk about Virtual Cables in this forum, use the search function to find them and learn how it works and where to download them

Hope this helps
Guillaume