@WTFE There was an article I read somewhere (I'll see if I can find it when I have time) that said the heating element might be unstable enough for copper particles to leech into the fluid it's heating, and thus will be breathed in (potentially causing problems) and breathed out (also potentially causing problems.) The 'phenomenon' of vaping is too new and the studies too poorly funded for us to know anything for certain right now.
The main point, though, is that smokers generally will move, or seclude themselves, or whatever. Not out of shame, but out of decades of developed decency. Persons who vape seem, from my own limited experience and with one or two exceptions, to want people to know they vape. And to demonstrate "ooh how cool" it is.
Just because something might not strictly-speaking be illegal (YMMV of course), like vaping on a train... that doesn't make it acceptable.