To an immortal, it probably wouldn’t make much of a difference. Either they die from cancer or they survive but then die anyways a few decades later. Plus, vampires have ways of extending someone’s life without any medical science anyways.

Though some lores for Dracula had him very deep and advanced in science as well as magic, so it’s possible he did have a cure for cancer but just didn’t care to share it because humans are just livestock to him.