What resolution is necessary to read standard text with VR? I'd imagine it would start to become usable for this at 4k per eye, and even then I suspect it would be limited.
VR headsets don't distribute pixels evenly across the entire view-space, the lenses concentrate the pixels toward the middle of your vision. So if for example you had a display that took up the center 25% of your vision in VR, it might have 50% of the actual display pixels. 50% of 4k is 1080p.
So you'd end up with a display that looks pretty large, but is only 1080p. Not optimal, but definitely usable.