Vertex color is one inexpensive way to add grime/variation, you basically just darken the diffuse texture by making certain vertices a darker color (white color = no change).
Vertex blending is similar, except instead of a colors you're blending in another texture, like a tiled grime map.
http://udn.epicgames.com/Two/VertexB...gTutorial.html
A second UV set can more than double the vertex count of the model, when exported to a game. So vertex color or blending can be a big savings.