This is starting to remind me of smoking in coffee shops when I was in grad school. I kept telling the managers that they would have more customers if they had smaller smoking sections with better ventilation and they kept telling me that people like to have a smoke after a coffee. "Look around, everyone in here is smoking." I would say, "Right, but I would be here 10x more often if it wasn't full of smoke."
So I go to Switzerland and come back and every single coffee shop on State Street is 100% non-smoking. What happened? One cafe remodeled and the owner noticed when they repainted how disgusting the ceiling was and, since she was going to add original art, she didn't want it ruined and decided to go non-smoking even if it cost her business. Within a month, there was a line for tables and other coffee shops were struggling - they immediately lost all their non-smoking customers. Within six months, all of the major college coffee shops in Madison were non-smoking.
I would say it's still to early to tell here. This could be a novelty bump that will go away. But maybe not. In families with one vegetarian, if they go fast food now (or last year), they were basically stuck with Taco Bell. If same-store sales are up every time someone adds a veggie burger, I think there may in a couple of years be few places that do not offer an alternative.
That said, for the reasons mentioned above, that might lead to them selling *more* meat, not less. And it will be interesting to see what happens to Taco Bell, traditionally the only vegetarian-friendly fast food restaurant.