If you think “high end VR” (HTC Vive or Oculus Rift), my rule of thumb budget is 1 Million to 5 Millions triangles, usually 2M-3M for a typical environment, depending on various factors.
And when it comes to texturing, I am usually trying to reach 2mm to 4mm / texel for high quality environments and 0.5 mm to 1mm / texel for objects details / closeups.
So, I would start simplifying to, let’s say, 2 Millions, texture it at 2mm/texel on 8k textures and see what you got here.
If the quality is not that good, you will need to simplify in a smarter way, using ZBrush, or MeshLab and playing with the smoothing and decimating filters to better organise your triangles.