Different id-adsense for different sections

[BirdOPrey5]

To the first question you can use template conditionals in the ad templates to show different ads in different forums, very easy to do.

For the second question, vBulletin forums can handle far more than 3 million hits a month- it's not the forum software that will limit you, it's the server specs and bandwidth which will be your limiting factor. If you got too big for one sever you could run multiple servers but 3 million hits is no where near the point you need to think about multiple servers.