New Laminate Plank Install Q's

First-timer here with a couple of questions as I start to plan
installing laminate plank wood flooring.

House is a on a concrete slab, area of work is about 650 sq ft.
Currently part of the work area is hardwood and the other is carpet.
These two areas meet together where the dining room and family room join.

1. On one side is existing hardwood plank flooring (dining room).
This flooring is glued down. Instead of ripping this out and dealing with
removing the glue, can I simply install the new floating floor on top of it?

2. On the family room side is existing carpet which we will be
ripping out. If the answer to question #1 above is YES that I can install the
new laminate plank wood floor on top of the existing wood floor, then I have a
small issue in that the height of the two (existing hardwood & existing carpet sections)
will be different. Looks like the difference would be about 1/4 to 3/8 inch difference.
So, should I put down plywood in the old carpet area so that it would be the same height etc?
Should I put down a vapor barrier between the concrete slab and the plywood?

Many thanks!