Pre planning is the way to as you have already suggested.
They are just being selfish by wanting everyone to mooch all round the streets until they find something they fancy to eat.
So either you pre plan for going to a good restaurants that caters for everyone or you split up to eat.
My mum is vegetarian and has been for donkeys years, I quite often prefer vegetarian food when I go out.
I'm surprised so many vegetarian on this thread still struggle to find more than 2 options when going out. Where do you live?
I've just googled the menus of 6 fairly local gastro pubs/ restaurants, all had good choices of mains from 4 to 6 , umpteen starters and sides. not one repeated same meal between them. (sorry I lie, 3 of them offers battered halloumi and chips)
I suspect some vegetarians are more like my DMs partner. Who is actually a fussy eater, so combine that with his veggie diet means he's never happy and believes there are not enough options for veggies. No he just doesn't like most of the choices because they have a spice in them or a herb he's never heard off, he will not try anything he suspects is too much for his palate. no matter how many times my DM says it's really really mild you would like it. he just pulls a disgusted face and says no.
We have all but given up going out for family meals when they visit now because he is a massive pain in the arse, and doesn't care that we are out to celebrate a DC birthday or mine and he just can not seat and eat in a place DC or me want to eat in for our special meal out, if the menu only has 2 things on that he could eat. He gets all uffy about them not catering for veggies(no they do, it's just the other things are things he doesn't like)
The last time we all went out for my Birthday, I researched a places that had won awards for catering for veggies, vegans, gluten free, dairy free and any other allergies you could think of. He sat an sulked because there was only 1 thing on the menu he would eat.