There's no way Apple would make a VR headset tethered to another device. To be on-brand it has to be elegant, which means all-in-one. It probably needs a new OS... certainly none of the UI from iOS can be reused, and it needs VR-specific heat and power optimizations, probably with a new low level graphics toolkit.
From Tim Cook's statement it seems pretty clear that they're going to wait until they can do AR glasses, which also fits their anti-nerd brand strategy. AR is more social, more human.
Although when the standalone Google/Oculus headsets hit next year I suspect people will be surprised how well they sell. The mobile VR app ecosystem should hit puberty around then. Assuming Oculus/Google can hit a $600 price point I think we'll see an iPod-like growth curve in that category, which would lure Apple into the market for sure.