Solid Hardwood, Engineered and Laminate Flooring - mobile home and veneer wood flooring install

I'm remodeling and have already purchased the flooring....I'm told I need some barrier between the subfloor and the wood... wouldnt the original linoleum floor be considered a vapor barrier? if thats what that stuffs for.
Also, I have a couple rotten spots in the floor... mainly at both entrances... I've cut out and replaced the plywood in the rear entrance, but the 3/4 ply I used is actually slightly smaller than the original. Ive heard years ago that there is a self leveling thinset of sorts that I can use to build it up flush with the original.... what is that stuff called and how much does it cost? (ball park)

First question is, how are you installing the floor? Floating? Pick up a portland cement based leveler at lowes or hd. Don't need anything fancy. If you're stapling/nailing down, use some asphalt saturated felt paper in layers to build up the thickness.

The linoleum should act as a vapor barrier, but you've got some spots that you've repaired, so if you're floating and you need a foam underlay anyway, spend the extra .15 s/f and pick up one with a vapor barrier attached.

hey thanks.... I'm floating it btw... so what does the foam underlay do exactly? is it for squeeky floors? or am I way off base

The foam underlayment gets rid of the "clicking" or "ticking" you would hear when you walk. A floating floor always has some movement no matter how flat the subfloor, and without pad, the wood would strike the subfloor everytime you step. So, the pad absorbs the movement.