This is basically a reputation system, isn't it? the newest version of vBulletin should have this built in, so it should just be a matter of turning it on and adding a rep-button to the visual themes.

I don't mind having both positive and negative reputation though, you can set it so that each user can only give a certain amount of positive/negative rep per day. This means you can allow each user to give for example 10 positive reps a day, while only two negative reps. This will ensure that people save their negative reps to the posts that actually deserve it.