Under which language is vbulletin created?
 

What is the computer language they used to create vbulletin?.

What tools, knowledge was used to create vbulletin boards?.
And how do you create vbulletin skins?

It's done in php mostly.

You create styles by modifying the templates and css in the Styles & Templates area of the Admin CP.

So php is a progamming language on its own?

Yes, php is a programming language. There is a lot of MySQL also used in the code.

Yeah, vBulletin is programmed in mainly PHP, using MySQL whenever any database connection is made.
It also uses some JavaScript/AJAX functionality, however not a major amount.

All of the above can be learnt at w3schools. A simple Google search will retrieve plenty of tutorials.

As Lynne said, you can modify the style by editing the CSS (or templates) in the Styles & Templates manager.

Refer to the vBulletin User Manual for help.