VRayMtl is missing from Material/Map browser can be easily resolved by selecting VRay Renderer.
How to set VRay as renderer?
Step 1: Navigate to Rendering -> Render Setup
Step 2: Then Click on “Common” -> Go to “Assign Renderer” -> click on “…” -> Select any of the “V-Ray renderer”
Do it for Production as well as Material Editor
Again to Material/Map Editor and check VRayMtl, it will be present over there.
If your problem is still not resolved, feel free to comment.