vBulletin has a built in "Promotions" system, you can setup a new usergroup with more permissions at 50 posts, 100, 150, 200, etc... But it isn't practical to have hundreds of groups- at some point they would top out beyond which there would be no more groups.
Otherwise you can simply check their post count manually and have them PM you or something whenever they reach a milestone.
Depends what the "reward" is- if its something that is part of the software or something you give/send them outside the software- like a T-shirt or gift card.
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