I encountered something like this. Are you using multiple monitors? I finally realized this happens when I had the VFR map or ATC window open in a different monitor. When I am not in VR, it’s really handy but with VR the second monitor is useless. Make sure you close the extra MSFS windows before starting VR - that worked for me.
FYI - I have not encountered the dim issue upon starting VR, but if I do, it’s nice to know ESC ESC will clear it up.