I just need to clarify here. vBulletin itself is allowed; we've got a list of users running verified legitimate vbulletins actually.
The -problem- is vBulletin actually costs a few hundred american greenbacks (dollars). When someone tries to use a version that hasn't been paid for, that violates US Copyright and Software Piracy laws, resulting in a permanent suspension. This is what has happened in your case - your version is actually tagged by the vB Smart release group - their own included "tags" clearly indicate it's been nulled, which is internet lingo for "hacked so it's free as opposed to paying the legitimate developers their well deserved money."
TL;DR? Legitimately paid for vB's are fine, warez copies like the one you got from a friend aren't.