I have a few more questions that would help in providing a recommendation.
What is the size of your group? (If you have 20-30 or more people, then you might want a place that can give you a private room. 6-8, then you're fine in the main dining room of any place.)
What tone do you wish to strike? A casual but luxurious upscale dinner that everyone will remember for a long time to come? Or would you be just as happy with, say, ethnic food in a not-that-fancy storefront?
And I'm not sure what to make of your reference to the food. I mean, the vegetarian part I get, but the comment about the grandparents doesn't provide much guidance.
FWIW, Italian food seems to often work with groups of diverse tastes - the vegetarians can get vegetable-based pastas, etc.