Question about copyrights and selling vBulletin hacks?

[K33]

Hello
This question is directed more towards vBulletin developers, however if someone else knows about the matter I would be glad to hear that too.
We have developed a big hack for ourselves to use on our website, however since it turned out to be so good :ninja: we are considering to sell it.
Our only concern is that we used portions of vBulletin code throughout the script, for example to manage attachements and stuff like that. Modification of vBulletin search engine, editing of some vBulletin functions, you get the idea.
The question is, are we allowed to sell it? Most of the code is still ours and script requires vBulletin to run.

Thanks in advance.