We have covered plenty of cool mini PCs here in the past. The ZOTAC ZBOX MAGNUS Gaming PC (EN1080) is ready for your VR gaming sessions. It is powered by an Intel Skylake Core i7-6700 processor and NVIDIA GeForce GTX 1080 8GB GDDR5X graphics. It is liquid-cooled, so it will survive your long gaming sessions.
The box comes with 3 x HDMI 2.0 and 2x DisplayPort 1.3 ports, so you can connect up to 4 displays. Not a backpack PC but still quite compact.