Animating Vertex Group weights and Shape Keys

Hi,
I am trying to create an animation by changing the vertex group weights while applying a shape key.

  1. I have a plane with a hight resolution.
  2. The plane has one vertex group calles MyGroup.
  3. I animate the weights of MyGroup with a vertex weight proximity modifier.
  4. This means that when a specific cube comes close to a vertex of the plane the weight of this vertex in MyGroup is faded from 0 to 1.
  5. This works fine if I use MyGroup in a modifer. For example a Displacement modifier.
  6. This does not work when animating from one shape key to another with MyGroup set in the Blend section of this shape key.

Does anbody know why this doesn´t work or if there is some kind of way to make it work? Or are shapekeys calculated before modifiers are and therefore can not “see” the animation of MyGroup?