On mars to your left on patrol theres a cave where vex spawn and meet with cabal and duke it out. Camp on top of the cave and take out the cabal from a distance and the vex respawn underneath you in the cave. Respawn is fairly quick because they are designed to spawn and fight each other. I did this by myself for a little while and got a few blue drops. With more people this can be done quicker and even a possible more efficent method.