Hi,
Unfortunately that's a limitation of the render engine: VRay's Displacement cannot be applied to instances or proxies (by using VRayDisplacementMod / Displace map within the VRayMtl applied on distributed items). Instead you can apply a standard 3ds max's "Displace" modifier to the source object. Just take in account that all scattered items will have the same displacement, since they are instances of an unique geometry.
Best regards,