Slab Floor Sub Base

[bdcool]

I have a 24x32 metal out building that currently has no flooring in it. i want to put a concrete slab floor in it. Currently the grade around the building is such that strom runoff runs into and back out of the building. I am going to regrade around the building and install underdrains. I am planning on putting a crushed stone base under the concrete with a plastic vapor barrier. Question is should I put the vapor barrier between the dirt and the stone or between the stone and concrete?