This time with ambient occlusion (thanks r4ptor for the suggestion).
I'm using vraylightmtl with a vraydirt material as override now. This seems to work great, but is there any way I can do this while rendering the final image? I currently render two images and put them together in Photoshop. It's no big deal, but it would be easier.
All I did was the AO now, so I did not fix any other bugs.
Nah. That's why I suggested you should apply to the ground only - it should be possibly to apply/blend AO on top of whatever material you have.
You could do that for all of your materials, but that's just trivial when you have many materials - much easier to do a clean AO render in such cases.
Or possibly a tonemapper with whitebalance adjustment. Kinda looks like it to me (vignette)? Doesn't look bad in this case, but you'd generally want The AO to be without colors or anything else affecting the render.