I'm no expert, but the key issue with cancers is that the body can't recognize them as a threat because the cells are its own. In theory, if there were some material that could train the immune system to recognize the existing cancer cells, it would fit the definition of a therapeutic vaccine.

I agree with you that an mRNA treatment isn't a vaccine, it's gene therapy. But on paper, a cancer "cure" using a traditional vaccine mechanism could exist. You'd need to come with some antigen that got it to go after the cancer.