One of the biggest reasons forums fail is because the founders didn’t plan to monetize it or were ill-informed on how to do it the right way.
Adsense isn’t just an ad platform. There are secrets to how it works and how to generate a lot of money for vBulletin and forums. I’ve been experimenting with Adsene for over 3 years, and have read numerous books and jorunalistic pieces on adsense. Furthermore, Adsense can be very rewarding if done the right way. Creating a forum is fun, but if you want to make thousands per month, you should read this guide about Adsense.