The link to activate is generated randomly, so you couldn't send it through this script (without significant modification). However, you could tell them to visit
http://yourdomain.com/register.php?action=requestemail to get another activation email.
This hack is specifically meant as an update email, so yes it is supposed to error if you haven't selected any forums. If you don't want that info, you could use the standard email function in vBulletin. (You can put their username and password in an email via that.)