the XML you find for templates, products, plugins, are generated by vBulletin itself when you export the tools you create... we, coders, never have to touch them really.
as i can tell by being a guru for a lot of useless things, i can tell you that when you are not into design, you need to know PHP with some OO twist, and MySQL when you explore database extractions... otherwise, PHP itself is enough...
i'm into design too, so i had to learn and apply a lot of XHTML/CSS2/3 before calling myself a cool designer...
and i find AJAX/Javascript very obscur...