Ardex Feather Finish, is what you want only use it where needed.
If you already have a vapor barrier directly under the 3/4 T&G I'd be reluctant to sandwich it with another for fear of trapping moisture. On the other hand, if dry now the only way that would happen is with a catastrophy, burst pipe or some other free water infusion, which would most likely be a homeowners insurance claim, so for product warrenty I would go with another vapor barrrier per manufacture specs.
Interesting note on a kitchen remod there was a improperly tightened connection on a dishwasher that resulted in flooding within a week of the Pergo install. Needless to say it wound up a replacement, but it took about 9mos. to resolve the matter before replacement. At that time though the Pergo had dried out when demoing it there was still free water trapped between the pergo foam underlayment and the previous sheet vinyl flooring. I though a pipe was still leaking. After allowing it to dry and having the plumber back again to reinspect his work we concluded it was from the original flood.
What I'm saying is it's your call, your floor. Go with the manufactures specs or if you feel your VB
is adequate go with it.
Another point, is the fiber board OKed for below grade use? We mostly used it in condos over liteweight or gypcrete floors. I'd check the specs.
Another point, I sealed my cut boards with pergo glue in potentially wet areas if your using a click together use wax on your cut ends an old candle will work.
I think that about covers it.