This sounds like the best idea I've seen yet for dealing with automated spammers. Here is some documentation on
Replacement variables from vBulletin.
This approach should limit automated spammers better than anything I've seen. Part of the problem is the simplicity of google searches for sites with vbulletin files and the register.php file.
Admin CP>Styles & Templates>Replacement Variable Manager> [Add New Replacement Variable]
Then you just have to rename register.php to your replacement variable to implement this simple spam blocker