PART ONE
  The starting point for this is to make a mixed material, channel 1 = basic material of the item, material 2 = the dirt function. To make the dirt function I make a custom noise or fractal function, with a colour map of dark colours with some lighter (for streaks) suitable for the material, I have a sepcial colour map for this, but it's easy to make one for yourself of dark greys and browns, with a few narrow bands of a lighter colour. You do not have to get complicated with this!! Simple stuff will do.

In the Mixed Material area, select "Color and highlight blend only" set to 100%, this adds only the dirt colours, so you don't need to mess with bump maps etc in the 2nd material.

  Now save your dirt material for use in this and other projects.
Next two images show you a normal texture map material and then the same with the procedural dirt function applied.

Material downloadable here! My Vue Material Gallery

PART 4: IMPORTING THE TEXTURE MAPS INTO VUE
  Some important points to note when you import your texture maps:

1) Do *NOT* load them as the default "Projected texture map", instead load them as "Texture map", it helps keep things straight with the UV maps. Also this must be done to the original material texture in slot 1 if it uses a texture map! Trust me on this ;) if you don't you'll end up with problems.
2) When you wish to scale any of the texture maps in slot 1 or 2, you must do so here in the material editor. otherwise they'll end up screwy. For example, if you connect the Bump map to the texture map, and alter the texture scale you'll see they don't add up right, if you haven't done as I suggest.
3) Make the materials "Object: PARAMETRIC" mapping so the UVmaps will align correctly to the model.

I like using a function so I have both procedural AND texture maps and can combine them or use one only at need, by using a "blender" function.