Dave Girard with a great post showing how to mix multiple displacement maps in Maya using Maya’s Utility Nodes, which would work for any Renderer of your choosing in Maya. Typically, building blended materials would be created differently in Mental Ray than it would be in V-Ray.
This process works for all renderers since it uses Maya’s utilities, nothing specific to V-Ray or mental ray
Dave shows you to achieve blends between multiple displacement maps by connecting the greyscale images to a Maya Plus/Minus/Average Utility and then sending the output of that Node to the displacement slot in the Shading Network. Check out the post for Mixing Displacement Maps in Any Maya Renderer here.