The VRay Dirty is a texture map that can be used to simulate a variety of effects, for example dirt around the vertices of an object, or to produce an ambient occlusion pass.

This tutorial will be focusing on the dirtmap material which is included in vray 1.5.  If  you use this material in your  renders they will have  a little more realism.

Setting up the Vray Dirt shader

Download the support files from here supportFilesDirtmap.zip
Load the scene file and create a new Vray material
Assign the material to the building walls (i have grouped the walls of the front building so the windows are seperate)

Add the VrayDirt shader to the diffuse slot

The default options are displayed for the vray dirt material.

Map a bitmap file texture to the radius slot and link the supplied dirt.jpeg file (set its U and V tiling to 2).

Map a bitmap file texture to the unoccluded slot and link the supplied diffuse.jpeg file (set its U and V tiling to 2) .

Test rendering the scene

Render the scene to get a quick preview

You should get an image like the one above, notice there is very little dirt
Increase the radius to 2m and render again.

Increasing the radius will make the building more dirty

Final rendering

Set the render setting as follows to produce a better quality image
Image width/height: 800 x 600
Image sampler: adaptive QMC
Antialiasing filer: Mitchell-Netravali
Adaptive amount: 1
Noise threshold: 0.005
Min samples: 8
Global subdivs: 2

The final vray dirt image

Final note:

You can plug in falloffs and shaders into the unnoccluded/occluded color slot to build more complex shaders when working with vray dirty material.