System Information
win 7 and gtx 970

2.77a
Broken: 2.77a abf6f08
Worked: 2.75a

particle object distribution using vertex emit from" not working as it should with vertex group

subdivide a plane.
in edit mode select some vertices
add to a vertex group
in object mode add a particle system using hair (advanced)
apply default cube as object to be distributed
set emission to match number of vertices selected in the vertex group.
under vertex group set density to use the created vertex group.