Serious problem going on for days now. I just reverted back to amember v3 because new members were being entered into the vbulletin database with the wrong groupID and thus could never login. Also had some other redirect issues. I reverted back to amember V3 and now cannot login even though members are now added with the correct groupid as they were. Other members are having some problems logging in as well. Only other change has been using vb 4.1.9 which I believe was working earlier. Suddenly things just stopped. Most users (I tested this) login to amember, they are logged into amember, but when they get to vbulletin they are not logged in and even trying to post and using the vbulletin login, they are unable to do so. UPDATE - Many more users are now able to login, not sure what cleared up. Apparently it's all the issues with vBulletin 4 which unfortunately nailed the forum badly for several days.
I've figured this out and sent it to the guys. Beware of v4 registering users properly but for some reason vbulletin (at least 4.1.9) is ticking the "COPPA User" setting to yes. There is no proper vBulletin setting to turn off all COPPA usage - only REGISTRATION. Pretty subtle. This doesn't happen for me with V3. So if you've had a problem with users not being able to login or do anything despite registered properly and seeing the user as "registered" in the membership field.
I've figured out a bug in the vb4 plugin and I've sent in the fix - it's an easy one. I'm still having problems with a login on my site which could be only my site but the main issues are solved and it looks like the plugin can be functional.