That can't be the answer, though. There has got to be a way that they can look up a record by the vanity tag alone. Otherwise the authorities would not be able to identify a car that had a vanity tag unless the owner was present and gave the necessary info to them. Kinda useless in the case of a hit-and-run incident.
But whatever the reason for the double-tagging, here's the really bizarre thing. You have to carry the numerical plate in your trunk, and if you're ever pulled over, you have to get that plate out and show it to the police so that they can look you up under that tag. Now, say you're in a distant state for the summer (oh, let's say California...) and you get pulled over. What cops in their right minds are going to believe that they should ignore the plate on your car and look up the other one that you've got stashed in the trunk?? You're going to be lucky if cops 3000 miles away have even heard of Delaware, let alone having any idea of how Delaware's DMV does things.
Suffice it to say that I drove with unusual care and restraint last summer.