I would like to point out that the primary demographic for movie-goers is already the younger generation. Childhood obesity, and obesity in general is probably at an all-time high in the U.S. right now. It's obvious that younger generations do think differently. They think less about their health when eating than do older generations. You will probably find that older people are much more careful about what they put in their bodies. Sure, Grandma knows how to spoil you, but she herself eats lightly and more healthily when you are not around.
Some modern ideas about what is healthy and what is not have proven wrong. Recent studies show that bacon and eggs for breakfast is actually much healthier than previously thought, for example (can't recall where I read it, sorry). Diet sodas, once heralded for the low calorie count are turning out to be at least as unhealthy as regular sodas, and in many cases worse for people.
I'm not saying healthy, vegan, or organic snacks won't sell to a "young, art-house, hipster, intellectual crowd", but I would recommend you find a theater who sells both and see how each kind of snack does in side by side sales. I'm willing to guess that the business staples of popcorn and soda still outsell everything else.
When you say you will only sell healthy choices, I think you may hurt your business, at least at the concession stand and perhaps with ticket sales as well. If you are happy only offering the healthy experience at the risk of losing income, then go for it.
You will need a large population base to support your business concept. In old posts on this forum, I read that art-house theaters draw 10% as many customers as mainstream. Art houses are said not to sell as many concessions to start with, so maybe it won't matter if they don't like your offerings. But, if I were operating an art house, I would want to sell as many concessions as I could. I would consider selling both kinds of food. It is not your job to police what others eat. Going to the movies is more of an event for most people, not an everday thing. So, if they want to splurge on calories or fat, or risk their health on non-organic popcorn, then that is their choice. I personally used to attend art flicks, but I've never been the kind to choose vegan or organic snacks.
I understand your concern for health, but I'm not sure people out for a night at the movies really care. For those that do, sell them the healthy stuff, for those that don't, sell them what they want, or they will go elsewhere.