well first off you gotta decide if you are going to buy a proper license or use a nulled version for free, or if you are going to use a free open source one.
If you are going to buy vBulletin go here https://www.vbulletin.com/order/
You could check out http://vbteam.info for some better quality nulled vBulletin, or http://www.forumscriptz.org is pretty good for vB, IPB and XF.
Good luck mate and by the way most themes and mods for vBulletin are explained very well most places so you dont really need to be worried about that.