I've experimented with the demo (https://blueimp.net/ajax/) and it looks good! Perhaps there are alternatives but should an implementation be pursued, this looks perfectly suitable (their statement of VBulletin intergration is promising)

code_glitch : I've been a fond user of IRC in the past but yes I agree, spamming is rife. Regulated IRC unless supported by some custom bot would add a layer of complexity to site admin tasks.

Such a solution as the proposed, given it could be configured to use VBulletin authentication; would provide an uncommon spam-vector and would have minimal impact on the sites administrative tasks (a ban in VBulletin is a ban in AJAX Chat). I'd wager that the uncommon combination of VBulletin + AJAX Chat is unlikely to be targetted by existing spamming scripts but then perhaps I under-estimate the persistance of spammers!