If the (problem) audio from Presenter needs to be present in the output mix from your mixing board, here's what to do.

1) Make sure that the Presenter/NDI input in vMix is not sent to the master output (M). Instead, only send it to either the (A) or (B) busses, assuming you're not using those for anything else.

2) In the audio output settings, make sure whichever bus you've used for the NDI audio (A or B) is going to a unique audio output device - not also used by the Main or Headphone outputs. If you only have one audio output device suitable for generating a feed to your mixing board at present, you may need to get another one (PCIe or USB e.g. Behringer UCA222). The idea is to send a "clean feed" of the Presenter/NDI audio out to the mixing board.

3) The return feed (main output) from the mixing board will now contain the NDI audio, and as an input to vMix can be sent to the main (M) output (plus headphones, if required), but of course not to the A or B bus which is sending to the mixing board. You should no longer have a feedback loop involving the audio board and the Presenter/NDI audio will be included in the vMix main output.

If you need to send a different audio mix to the vMix output, in terms of the NDI/Presenter audio, than that which is coming back in via the mixing board, then you will have to employ a mixer with a subgroup or aux bus output, such that it doesn't include the NDI/Presenter audio in the output that's going to vMix, and then you can send it directly to the Main output in vMix (as well as out via bus A or B) and adjust the level within vMix's mixed output independently of what's being done on the mixing board. However, the downside of that is that what you do in vMix will affect the level being sent to the mixing board - if that's a problem then there are more advanced solutions but they involve using virtual audio cables or a bit more external wiring/routing.