vMix does not support Blu-ray discs natively – it can play DVDs (and video files like MP4, MKV, etc.), but Blu-ray playback (especially protected discs) is not built-in.
If you want to play actual Blu-ray discs on your PC, you'll need a dedicated Blu-ray player software:
CyberLink PowerDVD - very reliable for discs(though pricey).
iToolShare Blu-ray Player – a great, more affordable alternative that works out-of-the-box.