Relying on VCs for all blending is very expensive for the engine. We use Vertex Alphas (VA) to blend additional dirt details into a model only when the asset is either a key 'landmark' or needs to be highly repeatable without holding up production for the same asset modified 50-odd times.
We actually reversed the blend where dirt is black and the standard diffuse texture is white. This is so it remains intuitive to the artist (dirt is dark).
PyrZern has a great method
if you want to be able to use the gradient directly in PS or another 2D program (and want it to match up) baking VCs OR VAs to a map - either through xNormal/Maya/Max onto a separate map & then combining within the texture is going to be your only real solution - getting something with a tone of UV islands to remain seamless when painting by hand is going to be extremely painful.