If anyone is using AdSense on their Vanilla forum, here is a plugin that will help increase your click through rate.
I'm not sure if it is still under active development or even if it works with the latest version of Vanilla, but I used it on a forum I was running before, and it increased CTR by 2-5% depending on the subject.
I actually recommended for this plugin to be made because I realized that if people are landing on a discussion from Google, it will most likely be because they are looking for information about the discussion topic. Meaning they are more likely to click the ad, especially if it is so conveniently placed within the discussion topic. Clever, no?