For my Moze, I definitely prefer purple Trackers that have a mix of cloning and bouncing traits for maximum Vampyr procs.
That said, I’ve had ok-ish luck finding purple nades (heck even the occasional legendary) in just normal ammo chests while playing on M4. During this vending machine event, my farming route has been in Floodmoor Basin with the game set to M4, going between Reliance and Knotty Peak to check the vendors. While I’m in Reliance, I open everything within eyesight of the vendors (including several ammo chests, a white chest, a fridge, and of course that red chest in the shed, which for some reason isn’t on a timer for me so it’s always available when I quit/restart.) Even the trash piles spew up blues and purples quite often.