Re: Disclaimer: I'm not a "concrete guy" 
Without having to pay for a subscription, did the article mention the reason for putting the vapor barrier on top of the insulation? Just curious.
No explanation, just
"The vapor barrier should be placed on top of the subbase, directly underneath the concrete. If you're installing rigid foam under the slab, put the vapor barrier on top of the foam." and
"For radiant slab, install 1- to 2-inch rigid EPS insulation followed by vapor barrier".
I don't have the answer, but it does seem reasonable to me. Put the vapor barrier against the concrete. Doing otherwise risks moisture getting between the barrier and the concrete, so why not put it directly under the concrete if you can? Diagrams from the DOE's Building America Best Practices Series also put the vapor barrier on top of the rigid insulation.
Example.
Can the risks be so small as to safely ignore the vapor-barrier-directly-under-the-concrete "rule"? Does placing the vapor barrier directly under the concrete make it more difficult to finish the concrete surface? I don't know.
That said, two normally reliable guys on
Green Building Advisor say it's OK to put it under the rigid insulation (my bolding on the first one):
The vapor (really a radon and ground-water) barrier under a slab should be either immediately under the concrete or under tongue & groove XPS with taped seams. Placing an impermeable membrane under the fill creates a reservoir for water (much worse with sand fill because of greater capillarity).
Best practice is to put the vapor barrier directly against the underside of the slab or under the foam as Robert has suggested.
BTW the article mentioned VaporBlock, Stego Wrap, Griffolyn, and Perminator as some choices for a vapor barrier.
