Basically:
Pay for vBulletin. If not, use shitty systems like phpBB, SMF, MyBB and other undisclosed clusters of "software." The only way to avoid being caught is to not do anything illegal in the first place. Did your parents teach you nothing? vBulletin have a very good anti-piracy team and I've seen them shut down entire servers just for one account running their software illegaly. Luckily for me I played things smart in the very beginning and bought vBulletin so I've never had the privilege of meeting their anti-piracy team for all the wrong reasons.
Follow Hoxxxy's advise - just buy the system. If you can't afford it, then you can't afford it. But copyright is serious shit on the Internet (mostly because the staff at Jelsoft lack social lives) and they will rip your head off and ..censored.. down your neck.