My truly stealthy setup is an 808 volt with 24 mg 100% PG unflavored liquid in a carto. I have a manual and an automatic one, and can slide it right up the sleeve of my shirt into the palm of my hand if I want to totally hide it. Small, barely any vapor, but hits like a train. It's the setup I will sneakily use on a plane, or in movie theaters or whatever.
My work and most of the time out and about "stealthy" setup is an eGo mini with some kind of tank like clippinwings linked above. It's a great vape, I just can't totally hide the vapor or the size, so I don't consider it quite as stealthy as a volt.