Correct me if I'm wrong, but isn't the server run on Linux? And since that's the case, couldn't you just setup a script on crontab to automatically check posts on a daily basis and delete them if they're a month old? I can't say I know much about vbulletin, but if it uses an actual database like MySQL running a command to delete posts more than a month old should be trivial. Even if it uses its own internal database system, a PERL script could parse the entries and do it with just a few lines of code.
1.0 Pastel Ball Python, 1.9 Normal Ball Pythons, 0.1 African House Snake, 1.0 Savannah Monitor, 0.0.1 Argentinian Horned Frog