He poured 4" concrete slab that he filled several inches with sand to
level

I wouldn't recommend the use of "Sand" for your concrete slab base. Sand
can wash out and isnt structurally solid unless it's contained within a
footing/foundation.
The norm seems to be a *compacted* layer of sand/mixed gravel (around here
they call it "road base"- other locale calls it Navvy Jack..a
sand/gravel/crushed rock type mixture used for making concrete). Thats what
I put under my concrete driveway and back porch slabs 5 years ago. No
problems so far.

As far as the plastic vapor barrier goes: code here calls for a tamped sand
base (within the foundation). Over that they place a 6 mil vapor
barrier..seams taped with Red Tuck Tape. Over that they poured the floor.
This is for "inside". Its your call if you want to use one outside for
your shed.
The gravel base should provide any drainage necessary under your slab (as
long as you provide a way for any sub-slab water/moisture to flow away) so
there shouldn't be any moisture concerns.