Sorry to butt in on this one and since it doesn't mention which 3D program you are using, I could be wasting your time.. But I use 3DSMax and for surfaces like a ground plane. I would use the 3DSMax Displace modifier.
Yes you need to have sufficient subdivision (polycount) on the surface to get decent quality, but the displace modifier, physically displaces geometry, rather than only at render time. Vray Displacement is a tricky thing to work with when you can't physically see the displacement, until you do a render. So I tend to use the 3DSMax displace modifier for simple stuff such as ground planes, water surfaces etc. and VrayDiplacementMod for trickier stuff, especially when the object has corners, because the vray mod, has controls such as keep continuity and tight bounds.