i mean its up to you of coarse, but i just don't see people thinking "ohh cool, lets post an extra 200 times so I don't have to press select all delete. I wouldn't think anyone would put any EXTRA effort into it. It might be a nice tiny bonus so its not as annoying, but why not just make it not annoying in the first place?
Also if you want to go ahead and do it, you should be able to do it automatic.
Say like you want to it it every 100 gives you 10, then what you have to do is make a new membergroup for having 100 200 300 400 500 600 700 800 900 and 1000 posts, and change the permissions for each member group for how many PMs they can have.
Well I just assume you can do it with vB, personally, I have only used vB for a limited amount of time and hated it so I don't remember a lot of it.