If asked about it by a smoker, I try to briefly describe why I'm glad I made the switch, what it's done for me, and how it's blossomed into a hobby that I greatly enjoy. I try to make it a point not to be insistent about how much better it is than smoking. When someone asks you what you like about vaping, I think it can easily be very condescending to prattle on about how superior what you're doing is to what they're doing.
I find people tend to ask more questions when I frame it as a preference. I.e., "This is what I like and here's why I like it." rather than "This is absolutely better and here's why." I think it tends to make the conversation more casual and comfortable for them. It's less alienating... ...for both parties, really.
And it's not dishonest for me to say that it is a preference.
I do tend to tell people that if they're ever interested in giving it a shot, I can help them get started for very little money. I always have more than enough vape crap for two people, just to have the peace of mind that comes with knowing that I won't be running out of anything unexpectedly or being left out in the cold when a piece of gear fails on me. If it's someone I know I'll be talking to again, then I will lend out a suitable setup to them.
It's kind of cool to watch someone progressively fall into that groove, just like I did when I went through the stages. I've wound up letting people keep gear I've lent them just because of how taken they've become with it. It's mindblowing, just how fast some people can become fully-fledged vapers and start pursuing the habit on their own.
I just tend to be against proselytizing because I don't like being preached at, myself. I don't want people to get the impression that vapers are overbearing and unapproachable. I think it's disrespectful and rather presumptuous to assume that people can't or won't make their own choices. I really despise people who pity those that neither need nor desire pity. At the end of the day, the only way they're going to pick up vaping is by their own choosing! It's their decision to make, not mine.
I will never be the one to initiate the serious conversation about vaping. Instead, I tend to make it clear that we can have that conversation if they genuinely want to. It's pointless otherwise. We'd both just be wasting our time.
One exception is at work. People have to answer to me there and I use that to my advantage a bit. I'm pretty lenient, but they have to ask me before they can step out for a smoke. The amount of time it takes to smoke a cig is a petty trifle to me, but on principle I can't have my crew always doing as they please with the time they're given there without my approval - it just gets out of hand with some folks.
However, I tell them that if they want to bring a vape setup along, then they're free to vape discreetly while they work - no permission needed. There's no policy against that. At this point, half of the smokers in the building vape when they're on the clock. Not much of a stride, but it's a start, you know?