Would hacking/spamming still be an issue if the user/password combination was a user who had absolutely no rights other than viewing forum posts?
Trust us you dont want to remove the built in securities of vBulletin. You'll have some idiot bot pounding at you forum 24x7 just trying everything to get in which could lead to more problems from the bot doing it other than just whatever it may post such as server loads which could get your account suspended.