Very approximately, your speed for minimum drag will be about 30% above your stall speed (it will vary with type and configuration, but that's a decent enough rule of thumb).
So there would be no benefit in flying slower than that "minimum drag speed" - you get there slower, and use more fuel in the process.
I'd be astonished if any modern airliner cruises, even at "long range cruise" speeds, below that speed.
Low speed buffet is the vibrations set up by the very start of the stall, as small areas of the wing start to suffer small flow separations. It isn't the stall, it's the first natural warning that you are approaching the stall.