pf: regarding membrane- the most important thing is the perfectly flat floor. It cost $$ for this prep but once it is done, you will never have to do it again. You can easily check the installers work by laying a 8′ straightedge on the floor and look for light under it. You should not see any light or low spots. Have your installer do it for you. He will be proud of his work and that you care. This is the single biggest thing with respect to E floors creaking.
Weather you use cork, foam, rubber is kinda a coin toss for your underlayment. Your budget will determine. Some E floors come with preattached 1/16″ high density foam to the planks. Others prefer using 1/8 rubber.
Vapor barriers under E floors are simple. Just heavy plastic with taped seams. Be sure this is flat too. No big wrinkles. No sand or rock poking through the plastic.
Btw, it is good practice to perform a moisture test on your concrete slab before and work is started. Better to find out if you have excess moisture now than after a new floor goes in.
