IMO, a vape-in is likely to be perceived as confrontational, regardless of how it is planned to go. Yet, I don't think idea of it being perceived as confrontational ought to be reason for not doing it. Of course, not all people would agree with that and conforming to accepted standards, however wrong they may appear to general society, is often preferable.
My idea of a vape-in would be to have a number of vapers show up at an establishment, and then split up. Obviously things would be coordinated a bit going in, but my idea is that if you had say 30 vapers in an indoor building, spread out and all openly vaping, but other patrons had 'no idea' that (vaping) was going on, it would send a very interesting message about how vaping indoors actually transpires, and how it actually impacts the non-vapers in that same building.
Additional ideas for this guerrilla tactic would be:
1 - have vapers who don't vape stand outside and educate, handing out flyers, cards and where to get more info about the topic, hopefully presenting both sides of the issue
2 - have other vapers who are indoors, but aren't vaping and who are secretly video recording the vapers who are. Thus proof that there were x number of vapers in the establishment vaping, while (presumably) no one noticed
3 - exit poll for non-vaping patrons to see if they noticed anyone vaping. Hope is that none do notice it, even while it is plausible that they easily could have. In event someone did notice it, it would be interesting and honest to capture their opinions about what they observed and how they reacted, plus if they noticed how many others were in the establishment doing the same thing
4 - deliver video/poll results to media agency or online medium that conveys what this vape-in intended to do and what the results were