I deleted my personal Facebook account at the end of June. At the time, I was using my preferred username: http://www.facebook.com/andrwmyr. 20 days after deleting my account, I signed up for a Facebook again. I used the same email as my previous account and all the same personal information. When I went to choose my vanity URL for the new account Facebook wouldn’t let me choose my previous username. I had to choose a new username.
I guess Facebook doesn’t attach vanity URLs to your email or other personal info. It looks like they attach it to your profile ID and nothing else. So, if you kill your profile ID you can get a new vanity URL.
If you want to kill your profile ID, this is how I deleted my Facebook account. And, Facebook’s blog post on usernames.
Update: Facebook now letting you change your username once from the “Account Settings” page.