I’m very sorry you lost your bun. I don’t think you caused it. Vaseline wouldnt kill a rabbit. Neither would fur mites. I dont know what lotion you used, and while lotion isn’t something that should be used on a rabbit unless a vet prescribes it, I’m thinking your bun had already been ill for some time when you saw the dry skin. Fur mites are opportunistic, most rabbits have fur mites in low numbers that just coexist with the rabbit without causing any trouble. If the bun gets ill and the immune defense thereby is weakened, the fur mites get the chance to multiply. This can happen fast.
If the vets suspected bone cancer, they must’ve had reason to. Its not a standard cause of death for rabbits. There would have been no cure for bone cancer.
I dont think you have any reason to blame yourself. I know how hard it is to lose a rabbit unexpectedly.
You loved your bun, and animals know when they’re loved.