Purcahse vB?
http://www.vbulletin.com/order
Setup? As in install? Or setup as in get it running exactly how you want? Install will be short and the longest process should be uploading the files. The actuall install itself only takes a few min. Getting it setup how you like may tak longer.
Security is a top concern for all forums based scripts, vBulletins track redcord for getting fixes out is under 24 hours, and with vBulletin 3.5.x upgrading is made easy by not always having to re-hack your files via the plugin system.