I'm testing my account and for some reason the free signup is not adding them to vbulletin, but is adding them to amember. The demo for actual payment is working, but not the free one.
Check amember CP -> manage products -> Edit Product -> vBulletin access Did you set it for all products?
Yes if it set to "registered users" for all products then aMember should add users to vBulletin. When you test with free signup, did aMember create completed payment for user? or just add pending payment?