I mean in each category or forum another targeted advertisement. I did not found any vbulletin mod for this, but i have been told that vbulletin already have built in advertising manager with ability to set different ads for diferent forums and categories.
vBulletin's built-in ad manager allows you to add ads in specific categories or forums and you can place it pretty much anywhere you want
Here you go.. Login to vbulletin administration and just go to "Advertising" > "Add New Add".