Depends on the person and the situation. If its someone I don't know offering unsolicited uninformed advice, I just nod my head in agreement without saying anything. Theres no point in explaining anything to them, anyone with common sense would research before making uninformed claims. Since they lack commom sense or are programmed beyond repair, I won't waste my energy or time with them. You won't get any of your points across, it will be like talking to a brick wall. If its someone that genuinely appears curious or is not condescending or snobby, and asks genuine questions and does not judge, I try to inform them to the best of my abilities.
If its someone passing me by doing the fake hacking lunger cough and giving me the look from a mile away from my vapor, I ask them if they are alright and need medical assistance in a very sarcastic manner. They usually ignore me but if they say its my "smoke" causing their cough I tell them its vapor and urge them to get their lungs checked out in the nearest ER, that something must be seriously wrong for a tiny amount of vapor to make them hack. If they get snobby I tell them to check out their vehicle for emission problems as they might be poisoning themselves and the enviroment with carbon monoxide.
If it is someone I know they usually don't tell me anything and the few that did were informed of the truth and never bothered me again, infact, some converted to vaping after getting the facts straight. Its easier to connect with someone you know about a subject like this