Registration is a known tedious process, and I believe vBulletin makes it very hectic for the average guest to register at your forum.
I believe all ADDITIONAL INFORMATION should not be there including forum rules. These things, members can fill out later and also read the rules later. After all, the rules are very similar from forum to forum and no members read them anyways.
Go to admincp > Style Manager > Edit Templates > Register Templates > register and replace everything with the contents of the attached file.
If you have a referral system, this modification isn’t for you. I think though, this will convert you more members than a referral system does anyways.
As far as I know this works on ALL VERSIONS OF 4.X and possibly earlier versions. I have been upgrading my vBulletin live sites with this modification and registration still works. I believe it also increases registrations tremendously. I’ve been getting anywhere between 30-60 registers a day.
Also it does include facebook connect and image verifcation ( even though the spam bots have beaten these)