[Honey]

This would have more than one con(s):-

1.By doing that the forum would start becoming a post ghost town..since there won't be much users who would keep there VPSs by posting monthly because first users would think of using there credits for keeping there vpses till the time it goes to $0(since in that situation we can't snatch the vps from them due to your explained system).

2.So this would make users more lazy, for example if you are making 20 posts with credits $5 each,then you would be excluded from current month's monthly postcount and also would have those 20*5=$100 which you would use them for next monthly postcount, in this way you can say we are allowing to half the number of monthly posts.

and there are many more..
so for getting excluded from monthly posts by paying your credits, would have too many cons than pros.
but if any other use for credits comes in anyone's mind then feel free to tell us.

Nah, If this system is really implemented, It won't be implemented in that manner.

First of all, I think If you want to implement It, you should reset the credits and set small amount of credits per post/topic etc, and 20$ for the whole month is too less, I'd recommend 1000$ / 20 posts, this would prevent users from using the credits to pay the posts every time, and this can only be used once to save the users If they are unable to post!