Then why not work out a cost for using the vBulletin license and a reasonable charge for using the server - a quick Google produced anything from $85 - $300 purchase price for vBulletin and $20 - $30 per month for using the server. Assuming a years usage I'm sure you could ask the seller for $500 - $700 to cover the costs you mentioned. Or were you considering something higher?
I don't mean to be rude, but why does it matter for you as an individual ? It doesn't !
You're asking us to help you do something that at least appears to me to be unethical, and possibly illegal. I'd say that the people here are going a long way out of their way to give you a chance to explain how this might be ethically justifiable.
Ok, here's what I understand it to be (my US law knowledge is limited).
1) You are NOT the domain owner, this belongs to the site founder, and thus he is at liberty to sell/lease as he sees appropriate.
2) YOU CANNOT be sued (as you're underaged), however you CAN file a lawsuit against any aggressors (this is where my limited US law knowledge may be at fault) but you may need someone to represent you.
3) The content of the website is yours (i.e. the machine it's hosted on, the implementation of the site code, & the vbulletin license).
Now, the things to determine are, what is being sold? The domain name? The site content? Assuming that you pay for the license, and have been responsible for the majority of the work (something that may have to be determined), then I would guess the "copyright" of the site belongs to yourself.
Also, the fact that you own/pay for the server and maintain it might give you the edge in the ownership war.
I'm assuming as the technical person you can limit access to the server code, and control at a root level? Thus your code (forum) is safe so long as you don't allow access to anyone else. If you do I suggest removing access until your situation is resolved, as you are protecting your personal work essentially (copyright).
At the end of the day if it's just the domain name that is being bought there's nothing you can do about it (as you don't have ownership of the domain), however if you own all the work (code) involved in the site then I suggest you protect it at a root level (by denying access) and resolve the situation.
persoanlly I'd send a registered letter to the domain owner and the potential vendors and remind them that they are free to buy the domain name (you dont' own it, but thats all they are buying/selling), and advise them that you own the rights to the db & applications software and you want some value form the deal. whether you own the rights to the postings is a moot point, depends on what you set out when users join.. they could be argued as open comment and non copyright.
the db, the server is yours and unless you are paid they won't get that.
more sneakily register a new domain (as similar as possible, or fucntionall as close a possible) right now, transfer the vbulletin / server to that domain. sahdow the new content on the old domain, wait till the deal is done, then deny any / all access to the shadow content. make sure your new server makes lots of references to the old site name, to encourage the previous users to migrate to your site. open sales negotiations with the new old domain owners, or their competitors.