[sturdavj]

A soil barrier is your objective.

If by a vapor barrier you mean a plastic type membrane (Visqueen is one brand type) they tend to lift through the rock becoming more of a problem than a benefit. Also they can trap moisture leading to a problem with dampness inside your shed.

Soil fabric in the way it is woven is actual porous allowing for movement of moisture thorough the rock and into the ground while at the same keeping the rock separate from the dirt. Also because of the weight the fabric tends not to left as easy, in any case be sure to place enough ( minimum 3 to 3 1/2 inches) rock over the top to weigh it down. You might want to check with a local contractor given the size of your shed a remnant size piece would do the job, good luck.

Not sure of your location, but in Oregon because of our rain fall everything we do is based on the elimination of moisture, this equates to mud which can swallow a yard of rock in about six months of heavy use/traffic.