In my situation, I am looking at layering like this:
=============== vinyl backed carpet
(?)----------------- .25" closed cell foam(?)
=============== Luxury Liner (vinyl side up)
-------------------- 1mm+ Firewall
-------------------- 1 layer Damplifer
_________________ sheetmetal
The .25" foam will probably only be in select locations, as things are already getting way thick without it, and the LL obviously won't go in the doors.
Somewhere in there, I intend to coat everything with Herculiner so that it is waterproof. I am thinking that the vinyl layer will give the LL enough structure, and Herc is flexible enough, that I can Herc over the LL. If this is not the case, I may herc over the Firewall (or I may just leave it in the can, since Firewall is waterproof)) and just lay the LL on top. The advantage would be that it could be removed - the disadvantage is that it could trap water.