as I can see your reply in email, but not here:
“Compress without alpha” is an option in the texture menu.
Vector Displacement Map should be very uncompressed.
no, engine/gpu/cpu doesnt work like that, unless you are doing UI (and even that is debatable) powers of two are way easier to calculate.
UE4 compresses because DirectX.
Also, if your monitor in-game will never be fullscreen, you can lower your texture size.