If Nahimic is not interfering with speech recognition, you can leave it running. If it *is* interfering, you can try setting the, 'Disable Acoustic Echo Cancellation' in Options > Recognition (and restart VA). If this still does not prevent Nahimic from interfering, there's not much else that can be done other than closing Nahimic, or try to figure out what configuration of Nahimic is causing issue (I would look for any voice enhancement options). I do not have Nahimic running here to test.
Software utilities such as Nahimic alter the audio stream in a way that the Windows speech engine cannot seem to recover (not all the time... usually with a certain number of settings). Wondering if this is something that Microsoft needs to be more aware of o_O.