Isn't this hair splitting on definitions? They are there for vaccinating against future instances of cancer after somebody is in remission from previous instances of cancer.

It's not a generally-distributed vaccine, but there are all sorts of vaccines that aren't generally distributed and instead applied only to populations likely to encounter the illness protected against (such as the battery one is exposed to if one travels to particular parts of the world).

They're treating the traces of cancer that are very likely left behind after the melanoma removal, that are likely to result in recurrence.