The "Powered by vBulletin" can infact be removed -- just that the copyright in the footer must remain intact unless you purchase the 'branding free' license option.
Here's how to remove it or change it to something you want.
In Phrase Manager, search for this phrase variable:
x_powered_by_vbulletin
Default is:
Code:
{1} - Powered by vBulletin
Change it to (or whatever you like) :