Hello Pancake,
I am so sorry your rabbit is poorly. The vet doesn't seem to be much use here, but I know from my own experience that the medical care of rabbits is an unknown science. I had a female rabbit spayed when she was six months old and the vet ar*ed up her operation. She bled to death and I still wake up having nightmares about it even now (5 years later). I took the best advice I thought I could get and still this happened.
The good news is that the uterus has been taken out, you spotted something was wrong and are obviously a caring pet owner. Your rabbit is six years old, which is a great age for a rabbit. My advice would be to give this little poppet the best quality of life that you can from this point on. I think the vet is probably just trying to prepare you for the worst, if that happens, as rabbits are such sensitive little souls. I wish I'd had my time again, I would have done things very differently. Let us know how bunny gets on, please? xx