What you require can be done via the ACP without the need for an additional hack.
In the
User Profile Fields section of your ACP, click
Add New User Profile Field and fill out the information.
You have the option to make it required at registration.
This information is then automatically added to the registration templates.
No further editing is required.
Check the vBulletin documentation for more details. It's quite an easy process.
Some info in this thread. Should be enough to get you started.
http://www.vbulletin.com/forum/showthread.php?t=205214