I've had the discussion about e-cigarettes with a few JWs lately, too. Mostly with the same result as you...they quickly change the subject because they know there's absolutely no logic behind prohibiting them.
As long as JWs are allowed to drink alcohol and injest caffeine, there's no clear logic against e-cigs or eating pot brownies for that matter. As long as it's not causing harm to your physical body, and it's not illegal, what else can they say? Any mention of "addiction" or "mind altering affects" can be easily countered by citing the addictive, and mind-altering, effects of alcohol and caffeine. And we know how much the average JW relies on alcohol and coffee breaks to make it through each day!