This issue probably only related to desktops and laptops with an Asus motherboard and Realtek audio drivers. Maybe it's been there for a while, or caused by recent software updates.

Recently we found on some of our PCs that adding a Photos Input caused a lot of flickering and stuttering when Fullscreen was also On. After many hours of investigations I nailed the cause down to Asus's bloatware "Sonic Studio". Judging from varios postings on the web it is diffucult challenge to remove Sonic Studio (and no uninstall is offered by Asus).

The fix I found is to disable the SS2UILauncer.exe in Windows' startup settings. (To find it, ctrl-alt-del into the Task Manager, and select the "Start-up" tab). Don't forget to reboot after disabling.

Side note, I am not sure why an audio thing affects vMix in this way. My best guess it's related to how Asus manages interrupts on the PCIe.