I think this is all down to good old common sense. A little whisp of vaper should be tolerated but blowing massive clouds, be it a bus, train, plane etc is a big no no.
Unfortunately, common sense is down to the individual. vapers and non vapers don't all possess this. There's always going to be someone taking the piss, chugging on a smok 365 turbo ninja tank at 200w, pissing people off and worst of all, causing bad press. Even that would piss me off.
That brings us back to regulation. The dumb ruin it for the rest.