After removing vinyl on luan underlayment in the kitchen, to my horror
I found 70's vinyl glued directly to the 3/4" plywood subfloor (over
2x10s 16" O.C.). I already have porcelain tile ordered and want to
lay 1/4" hardibacker + tile to match height of 3/4" hardwood floor in
the adjacent living room before installing cabinets. I am remodeling
to sell in a few months, and I don't want to get too complicated or
expensive (who does?).

The vinyl has brown adhesive or something troweled into its embossing,
but it did not actually stick to the underlayment above it when I
removed it--maybe from a previous floor? Except on its perimeter and
where the cabinets were and it was not adhered, the whole thing is
glued hard as a rock and does not come up with a floor scraper--the
vinyl will crack before it will peel up. Where I chipped it up, the
felt underneath barely scraped up; it's like concrete. I'd need a
razor to lift it. Also worried about asbestos--I sprayed with
detergent water during my removal trial, but I fear the whole thing
will be a real mess to remove because it might require power
equipment.

So I'm thinking: leave the vinyl in place and calling it an
"uncoupling membrane." Hardi doesn't want you to bond to the subfloor
anyway--the thinset base is just to get full contact & fill in the
gaps to avoid flexure, right? Most of the warnings about leaving
vinyl under tile are about underlayment and poorly bonded or cushioned
vinyl, as far as I can tell, which doesn't seem to applly in this
case. With effort, I can leave a fingernail print in the embossed
vinyl, but it feels like hard plastic and doesn't give--hence the
difficulty in removal. How's that different from schluter-ditra?
I've also read opinions to use latex modified thinset to lay the
backerboard over the vinyl--typically, I'd use unmodified thinset
under the backer (to prevent coupling with subfloor according to
Hardi) and latex modified for the porcelain tile. But if I am to use
latex modified on the vinyl, do I switch to unmodified at the old
cabinet outlines where the vinyl can be removed and it turns to bare
plywood?