Only Signature is not a way for getting back links you got from Visitor message profile Links and Image links. when you focus on Signature you need to posting for that it is not easy way. Most of signature give you permission post direct Links end of your post.
You can put a link in your signature. We don't stop that. What we do, however, is indicate in the HTML code that displays the forum that the links are "no follow". In basic terms, this means that while people see and can use the link, the search engines ignore it as if it weren't there. (for the most part). That means from an SEO perspective (the topic of this thread), you don't actually get a backlink. This is true in the signatures, posts, visitor messages and anywhere that you use the vBulletin tags to add links.