When I tested this, I was able to run the "Delete Vertex" command on the individual terrains. I noticed that it did update the complex terrain as long as I had the Merge/Append setting set to "Merge". If I had it set to "Append" it would keep the original triangulation from the point cloud.
So, here are a couple of questions... 1.) Are you able to run the Delete Vertex command on the individual terrain? and 2.)Do you have the terrains set to Append or Merge in the Complex Terrain?