If vBulletin was free to trial it, could you like use its templates and skins on other types of forum software? I ask this because I've actually seen IPB forums using a vBulletin skin before.
IPB wouldn't use a vbulletin skin, maybe a designer made an IPB skin to look like a vb one, but they are not cross compatible like that. With that being said, any design can be mimicked and can work on external sites. hope that answered your question