Doh, that's a tough predicament!
You are all set with a facebook page that equips a business name as your vanity url. As how page category works, page username can never be changed after this. You will have to delete the fan page and start all over. But as much you can fix it with the same page name, its old username can never be retrieved.
How to Transfer Profile Username to a Facebook Page | Best Facebook Resources
My problem was that I assigned my personal user account the name that I wanted for my fanpage... I follwed the instructions above to assign the name to my business page instead.
I am kinda wondering if you will have to re-create your business page from scratch?
You might ask your question on the Facebook forums. Don't listen to me, because I don't use FB much outside of my day job.