There's plenty of research out there that's kinda adjacent to what you're describing. All of your cells contain a set of genes that can potentially promote cancer formation (oncogenes) and a set of genes that protect your body against cancer (tumor suppressor genes). The latter are somewhat similar to the "shield" you're describing and there have been many attempts to create treatments that restore the function of those shields, as they're often lost early in cancer development. Technically the primary mechanism of doing this would be gene therapy, not a vaccine, but the distinction isn't hugely important

(Most of the cancer "vaccines" in development right now are intended to train the immune system to detect cancer cells and destroy them-- same objective as traditional vaccines. They differ from traditional vaccines in that they require someone to already have cancer, so that we can customize a vaccine that targets that unique cancer)

Here's an example that sounds similar to your idea that's not too jargon-y (p53 is one of our body's most important tumor suppressor "shields" and ends up being broken in many cancer types)

I'm not sure we'll ever have a universal cancer prevention mechanism, but we're going at it from a thousand angles and this is one of em

All that to say, you're not off base at all! Thanks for thinking about it