How do I remove an addon domain from one vndv account so I can add it on another? I created one but I can't use the database because it's more than 16 chars so I want to create a shorter vndv account, but I need to use the domain again. . any thoughts?

go to your web hosting account and access the "Add A Custom Domain Name" section. You should see your domain and the option of deleting it.

BTW, you can just create the other account for the database and still use the first web hosting account. This way you don't need to switch the domain to the other account.