Lately I guess my personal stealth vape is my sai. It's no bigger than a fury2.
But it should be carried or stored upright. At least if its really loaded, and the load is liquid. I haven't had mine leak, but I did have rosin flow to one side and harden there as it cooled.
Today I had it in a cup that I had in my mowers drinkholder. Not so much for stealth just for a place to put it while I was cutting grass.
One reason I consider it stealthy is the time it takes to have a session. Typically for me it's 2-3 long inhales. In maybe 2 minutes I'm done. And since it's concentrates it lasts a long time as well. There no real prep if it's loaded, just press the fire button and inhale.
Another is smell. It's not close in odor. I can open my truck door take 2 hits, put the sai away, and go into a theater, standing in line without any sideways glances. And enjoy the movie a little more.