3ds Max :: VrayDisplacementMod With Mixed Amounts?

I have a problem with the VrayDisplacementMod. I have a large green area covered with grass ( a golf course ). There shoud be 3 types of different grass colour and hights. I have made my grass using the VrayDisplacementMod and received really great ( as for my needs ) results . Also have used the vertex map for the cool mix for the grass as the main gras in the middle is a yellowishgreen when the rest is covered by the darker grass ( around the main one ) . There is now really nice effect bettwen these 2 grasses but the problem is that the whole area has the same amount of VrayDisplacement....

I mean to get the effect where the dark graas around is much higher than the other one (in the middle of the area ) but I would like the grass getting smaller in a unnoticed way so that we could NOT see the border line between the hights ( just like the colour using the vertex map )