If a user changes his password in aMember, this change is not reflected in vbulletin. If a user attemps to login in vbulletin, it will fail.
New users can't sign into vbulletin at all. Looking at the "user" row in vbulletin's MYSQL database, it's completely blank.
Solution: I rolled back to an old version of the plugin (amember side) found in: amember\application\default\plugins\protect\vbulletin If anyone else is having this issue, ask support for an older version of the plugin.
(sorry for all the posts, this forum won't let me edit) Follow up: I opened a ticket and aMember fixed the plugin while I slept As always, great service from aMember.