Are you doing a glue down or a floating floor? You've used both terms here. A plain glue down on slab is standard, or floating over a vapor barrier/cushion is standard. Pay attention to the substrate for either. It should be FLAT. No more than 1/4" variance over a 4' distance. Grind down the high spots and fill the low spots with SLC.

Is there a worthwhile advantage for putting a vapor barrier/cushion under a glued down engineered floor?

The glued down barrier will cost an additional approx.$3 per square foot more., and I was just wondering if it would be worth it.

My neighbor used a cork underlayment between their glued down engineered wood floor and concrete slab; we had our floor glued directly onto the concrete. I notice no difference in sound or feel when walking on their floor and on ours. Also, my neighbor has complained about how cold and hard her floor feels and how it aggravates her knees. Neither floor sounds hollow...they're not floating floors, they're glued down. I'd like to emphasize strongly what livewireoak said about the importance of a flat substrate. Apparently, our concrete substrate was not flat, and the boards did not stick (they are 5" wide planks which makes it even more important for a level substrate as they won't "give" like a narrower plank might). After the installers returned for the third time to drill holes to inject glue because of non-sticking planks (we counted 90 drilled holes), we said enough. We hired a floor inspector and he showed us with a 6' level that some areas of our substrate had a 1-1/2" variance, which was why the planks were not sticking. We demanded a new floor and the builder complied. But it was a mess pulling up that glued floor and leveling the concrete.

A wood product can not come in contact with concrete. Is it engineered wood or a laminate? Neither can go down directly on concrete.

Engineered wood is glued to a concrete slab 90% of the time here in the South. The other 10% is a floating install over a vapor barrier cushion combo. Gluedown slab on grade is why engineered wood was developed in the first place. It was designed with that very application in mind from the beginning. With the proper adhesive, and a flat slab, it's not an issue at all.

Laminate also is installed directly over slab on grade with the combo moisture barrier cushion as well. But since laminate is never installed without that moisture barrier/cushion, I'm not sure what your point is for that either.

Lets take a look at the glued down wood product in around 10 years. Some engineered flooring has a moisture barrier on the back of the product, that application I agree with. Our codes here won't allow pressure treated lumber to come into contact with concrete.

It"s been done for over 30 years around here with no issues unless it's below grade with active water infiltration. And they've been installing plywood to concrete with solid wood stapled to that with only minimal problems in high moisture areas. Really. It isn't a problem. I have no idea why your local code is what it is, but it seems to be severely over reactive for some unknown reason. Especially considering the long successful history of doing exactly that here in the humidity capital of the middle South.

Over reactive? Man I live in Canada, I'm surprised they haven't come up with a code for the proper side of the bed to get out of! A permit is needed to change a toilet.

Below grade is probably the issue, but practice always is to have a barrier of some type between the 2 materials. Acoustic sealant is always used for sil plates in garages for example, that's on grade. I am going to see my GC pal still in business and ask. I wonder if it's the moisture and or chemical make up of concrete in Ontario?