On a slab there needed to be a vaper barrier before anything went down. I never would have used wafter board on a floor like that, The moisture will make it fall apart.
I would have also used Allour flooring, it's a floating vinyl strip flooring and would have needed no glue at all.
What type do you have? Has anyone taken the time to read the directions on the box to see what's called for.
Also there's no such thing as vinyl hardwood planks, there either hardwwod or vinyl. They may just look like hardwood.