I wasn’t aware the word ‘vaccine’ was used in this sense.
Technically, a vaccine “teaches” B-cells to make antibodies, regardless of whether those antibodies protect against disease or not. Antibodies recognize proteins by their shape and electric charge and then attach to them. When antibodies attach to a protein, they signal other components of the immune system to act; in this case, the other components of the immune system would attack the cancer cells, since those are where the antibodies attach.
Although this approach could work in theory, I am highly suspicious of the claim. Cuba is not exactly a research powerhouse, and I doubt it has the resources to adequately test such a drug. Also, since antibody therapies can cause quite a few unintended side effects through their effect on the immune system, I would be wary about running off to Cuba to take this treatment.