But vertex displacement needs hi-poly goemetry/subdivision to work and when you subdivide to high levels VRAM usage goes skyrocket with vertex displacement while regular displacement you can displace a single 2-poly plane.
Not sure why is this vertex displacement recommended in every topic when someone asks about texture displacement issues, frankly i still haven found any proper usage of vertex displacement in arch-viz/sea or similar cases since it requires huge amountn of VRAM to come anywhere close to nice texture displacement textures, especially on close shoots like terrain, pavements, waves.....
I'd rather see them spend more time on updating "old" texture displacement to fix limitations and make it faster than this vertex displacement thing which currently is very limited usage.
So i understand initial question very well since i've meed those problems too in past and vertex displacement wasn't a solution, well at least not for my big projects (i can't spend 3-4 GB VRAM just on subdividing sea polygons to work nice with vertex displacement)

.
BTW Beppe sorry if it sounds like i'm lashing out on you, it's not really, I know you are just trying to help and thanks for all of your great help and suggestions on the forum, do you ever sleep

:)?