Monitor from the actual PC headphone output, thats the vmix output and is a better representation of what is going out. Also, keep an eye on the level meters on the output monitor.

It depends on whether you've got the right sort of mixing board for this type of job. You need one where you can set up a monitor mix (for your headphones), fed via a post-fade send on your mic channel(s), which is independent of the main output (or whatever output is being sent into vMix). You would also need to use one of the A or B aux outputs from vMix to send everything else that's in vMix's main output mix into a spare channel on the mixer, and use a pre-fade send of that signal into the monitor mix. This would need an element of calibration to set the monitor mix so that what you hear will exactly correspond to what is going out via the vMix main output. It might be worth hiring an audio engineer (with the necessary test equipment) to come and set this up for, if your mixer has such capability. Alternatively, you could use a second small mixer, with a couple of inputs, to perform the same function, generating a calibrated mix of the main mixer output and the other audio from vMix.

However, rather than depending on what you hear, I'd also suggest the use of audio metering that can be placed near your eyeline, be that software meters on a screen, or traditional hardware audio meters (taking a feed of the main output from vMix). You might also wish to experiment with my NDI NoAudioMeter software, which provides a "traffic light" green/amber/red display of audio levels, with adjustable thresholds between them, and can generate Low Audio alerts in a number of ways, including audio bleeps, relay control (to switch external lights), or via an NDI signal.