Hmmm.
I have no idea how it could be done, simply, that is.
vBulletin doesn't log (in MySQL backend) instances of member post counts. Not to say this can't be done, but it would require you to keep track of each member's post count per day.
Than you'd just have the user input a date, and return all matching post counts that fall at the date (or between two dates).
It's an interesting concept for a hack. Until vB3 RC is released, I wont be doing any serious hacking.
p.s. that's not saying I plan to do this, but might consider a variation.