There are many ways to fight DDoS, not all good at it, but it is always server thing, not a web application. vBulletin is web application.
They could be hardware ones, but also they could be software ones. For example, litespeed, especially commercial versions are advertised as good DDoS defense. Didn't tried this. You also have apache's mod_evasive, but to be honest, I'm not thrilled with it's performance.