Technical fog effects are extremely intensive on mobile and VR platforms, so avoid using atmospheric fog and exponential fog components where possible - take it from me, you’ll be fighting against every other aspect of your project to maintain frame budget. Instead, try faking a fog effect with this: UE4 Tutorial: Multi-Colored Fog (Journey) - YouTube
If its volumetric fog that you’re after, I think you’re out of luck. If you can get volumetrics to correctly render in VR, you’ll still struggle against the performance impact.