If they legislate ecigs in a certain way it's going to require some element of proof of it's harm - and something being similar enough in appearance is not justifiable, otherwise drinking juice in the streets would be the same fine as drinking alcohol in the streets.

The whole thing is dicey. Ecig vapor is primarily water vapor. Potentially if you smell it, you are smelling flavorings - esters - similar to the types of chemicals used to give perfumes and such scent with one small difference - those chemicals which give the ecig their flavor are all food safe, and are the exact same chemicals that are used to flavor everything from cake icing to cookies. Some may argue whether or not you are getting any sort of trace nicotine as well, but to be honest, the quantities are so abysmally minimal, that it's more than splitting hairs at that point.

To give you an example from my own calculation of what I vape:

My ecig juice is 3mg strength. This means that for every 1L of ejuice, there is 3mg of nicotine. I use a medium sized pen type of setup. The average amount of juice that is converted to vapor in a puff is around 10mg (1/100th of a gram). Of that 1/100th of a gram, 90% or so is vegetable glycerine, which converts to water vapor, and 9% or so that are esters (flavorings). The rest is nicotine - or in my case, 0.0003mg of it. Less than the amount in a tomato, according to the New England Journal of Medicine.

Now, the majority of that laughably tiny amount of nicotine is absorbed in my blood-stream, so if there is any left during an exhale, we are talking about such trivially small amounts, that the equipment to even detect it is hard to come by.

And during all this time, 20 cars have driven by and you've inhaled all sort of carbon monoxide, soot, and god knows what else.