Or instead of even a secondary button, just do what Poser Fitting Room does, which is that you have a brush that lets you paint the vertices (to decide how much influence the simulation will have on their behavior) and this will apply a value that is pre-defined in an edit box. The default value could be 100% influenced by the simulation, but the user can then alter that value to 50% or 0% or anything else. There could be two ways to paint the vertices: a regular paintbrush which applies the value exactly, and an airbrush where the paint is applied exactly at the middle but the outward paint will cause vertices to have a value that is slightly less than than the specified value.

There could be another button that will take whatever vertices are currently selected and expand the selection outwards (similar to the "Simple" button in the vertice tools) and with each expanded selection, the VWD tool will decrease or increase the simulation influence by a value defined in an editbox e.g. -5% or +10%

Just ideas... it would certainly be nice to have more control over which vertices are affected by the simulation and by what amount they are affected.