If leaving out the part that vaping can be addictive (as a hobby) and costly, then not so fair. If upfront with that added information, that you are aware of, and that likely comes into play with full switch, then I would say very fair. If looking just to add notches to your belt on conversion and not really caring what happens after the switch is made, then it is, in general, a fair proposition.
I hold no information back. I tell them how much the vaping hardware costs - and that it'll need replacing more often than they expect, being made in China, often with Chinese quality - I tell them how much pre-made juice costs, and I tell them some things need replacing now and then as a routine part of the vaping activity. They know how much it costs to vape pre-made stuff, and they know full DIY (juice and attys) is about an order of magnitude cheaper if you care to get into that.
For a 20-a-day smoker trying to get his kick with vaping, I tell them a good starting point for cost calculation is 3 ml of 12 mg nic juice a day, and one atty head per fortnight. How do I know? Because that's a little more than what I do in the same situation (although I don't do the atty head change thing anymore, since missus told me to lose the clearos).
But that's *on average*. The guy in question went from being an average smoker to a massive vaper with elitist tastebuds. He changes atty heads every 2 days. It's crazy! And the 1100 mAh battery I sold him lasts him less than a half-day. How was I to know he'd get into vaping as much as he did?
As for trying to convert as many people as possible by deception, why would I do that? It's more trouble than anything else to me. I do it because I clearly see those who think about switching are totally confused and intimidated by the vaping world, what's available and what to expect, and as a result, don't even dare push the door of the vaping store. It pains me to see someone who's about ready to kick the habit of smoking and doesn't dare take the first step, because I know what I've gained, and I would've like to have had a little help myself, instead of discovering everything on my own and making mistakes.
What I provide - but only for friends and people I care about - is a starter kit I know works well for a beginner, at a reasonable cost, a bit of friendly advice to get started, and more importantly, the confidence that they can try it without feeling guilty for having blown money on something they might not like in the end. It might not seem like much, but a little personalized help from a familiar face often makes it much easier to get into something completely foreign - and that's not just with vaping. Lying to get them into vaping, even if it's for a good cause, doesn't really fit the idea of giving good advice - not to mention, they'll know you've lied eventually.
So, I just provide an entry into the vaping world. But once they're confirmed vapers and they've paid me back, I'm happy to help if they want me to, but essentially they're on their own. I'm not Mother Teresa.