6 mil or thicker plastic with seams taped on the ground would be a good vapor barrier.is the crawl space unventilated? Then seal up any air leaks to outside, including around exterior cellar door if ya have one, and insulate the inside walls of foundation. If an unventilated crawl space is really sealed tight you'll wanta find out about the radon level in your area and install a proper vent for that.
Unventilated you want to, seal up any air leaks to the house, holes for wires, pipes, drains,ducts, etc., insulate the ceiling, or under the floor above, between the joists with bats, vapor barrier up, make sure its against the floor and secure it so it won't sag. Use wrap insulation on water pipes, cold and hot, possibly even electric heat tape on them, ( I've even heard of exposed sewer lines freezing) insulate any heating and AC ducts.
Have fun under there. Remove jagged rocks any other junk first, ya don't wanta hurt yourself or get holes poked in your nice new plastic.
Mine has a vapor barrier on the floor and insulation between the floor joists. I really don't know why it was done like that though, it's open to the regular basement and stays pretty warm in there in the winter. I have a feeling someone just went "ah insulation, great, we'll stuff it up there".