IPv6 support
 

IPv6 is not supported yet in the current vBulletin version because only the first 15 characters of an IP address are used. This is no problem for IPv4 since 15 is the maximum. But IPv6 addresses can be 39 characters long.
If someone can make a mod that extends the 15-characters limit to 39 characters, till would make vBulletin IPv6 compatible.

Thanks in advance for making such a mod for vBulletin.

Just changing the columnin the database to hold larger values might already do the trick.

construct_ip_usage_table() will break likely it appears.
construct_ip_register_table()
construct_user_ip_table()
fetch_substr_ip() might be an issue.
/**
* Verifies an IP address - currently only works with IPv4
*
* @param string IP address
*
* @return boolean
*/
function verify_ipaddress(&$ipaddress)

There's more I'm sure.

It's not that easy as changing one field. But hey, what do I know?