Windows 7 64bits
Broken: Blender 2.69 r60995 (official)
Short description of error
Vertex Weight Proximity Modifier (VWPM) seems to have an inconsistency behavior over Particles Vertex Groups Settings.
If I put it over the Length slot it works as expected (with realtime feedback). But the VWPM have no effect over the Density slot.
If I enable "Use Modifier Stack" inside Particles's Emission Panel, the modifier works over the Density slot, but with no realtime feedback.
After any change to VWPM, I have to disable and enable again "Use Modifier Stack" button to update particles and see the changes.
I made a test sample file: