Win 7 64 bit
When controlling a Hair Particle System by Vertex Groups, the appearance of the particles don't refresh automaticlly after doing changes in the Data Buttons -> "Vertex Groups" and if the Vertex Group is allready selected in the Particle System, to control the particle density, for example.
In this case I need to go to the Particle System afterwards and do select the same, already selected Vertex Group again, to get the correct 3D View apperence.
However, this is not necessary when I do changes on this Vertex Group by using the Weight Paint Tool. Then the Viewport appearance of the particles refreshes instantly.