How does the Vertex Weight Mix Modifier Work Internally I've gone through all of the documentation and been fiddling around with this modifier for an hour.  Can someone tell me how it works?  What is it actually doing?  My understanding right now is that it mixes different weights (which you set by vertex groups) and I understand all of the blend modes and what not).  Essentially, I understand a lot of what it's doing but I can't wrap my head around this problem.  In the vertex groups you set a weight for your vertex group and then in this modifier you have an A and a B group and you set default weights for the A group and the B group.  What is the interaction between these different weight settings?  How does it work together?  Also, how would you use a texture map on this (I looked up the blender post for this at Remove vertices based on texture but couldn't figure it out).  

Thanks so much for the help!
Have a slow read through the manual page on the vertex weight mix modifier, it has an example at the end to help explain it.

The points I think are most relevant -


  Vertex Group A
      The vertex group to affect.
  Default Weight A
      The default weight to assign to all vertices not in the given vertex group.
  Vertex Group B
      The second vertex group to mix into the affected one. Leave it empty if you only want to mix in a simple value.
  Default Weight B
      The default weight to assign to all vertices not in the given second vertex group.
  
  Mix Set
      Choose which vertices will be affected.
      All vertices
          Affects all vertices, disregarding the vertex groups content.
      Vertices from group A
          Affects only vertices belonging to the affected vertex group.
      Vertices from group B
          Affects only vertices belonging to the second vertex group.
      Vertices from one group
          Affects only vertices belonging to at least one of the vertex groups.
      Vertices from both groups
          Affects only vertices belonging to both vertex groups.
  
  Texture Mask
      An additional texture, the values of which will be multiplied with the global influence value for each vertex.


So Vertex Group A is the group that gets the result, you use this in modifiers after the mix.

The mix mode defines how the two values are mixed, while the set decides which vertices get the weight.

The texture mask can then limit the result to certain areas of the mesh.