I'm surprised plate-to-VIN information is publicly available for Washington state, at least. The DOL here will let you renew tabs online[1] using just the plate number and last four digits of the associated VIN.[2] I'm sure there must be some interesting fraud and/or harassment potential for being able to access someone else's account.
Edit: just to be clear, I'm not saying there's anything wrong with building an app that uses the data. I'm surprised the data is available from the DOL, given that they use it for authentication.
[1] And update one's mailing address information, AFAIK.