Heaven is an old benchmark. It stresses the GPU core, but doesn't use anywhere near 8 GB VRAM on max settings. You need something that does. You could try Superposition on 4K/8K settings, though it doesn't stress the GPU core as much as on 1080p Ultra for some reason, leading to lesser heat generation. Hopefully it gives you clues of a faulty VRAM anyway.
- thinking out loud
It fixes GPU temps, but doesn't do much with the VRAM, unfortunately. Your fans don't spin so hard to keep GPU temps in check, so you have lesser airflow going towards the VRAM and other components.