There's still a lot about cancers as a whole we don't know about.

The majority of them seem to be involved in the breakdown of DNA. So a vaccine against a virus (that affects DNA) would make sense.

But we also know about a large number of carcinogens that seem to break down DNA.

That said, the key word above is "seem".

I strongly feel that cancers are right now a "catch-all" and we're going to recategorize them in the next few years.

That all said, it's an incredibly novel delivery mechanism.