Other ideas and hints
Vegan is vegetalien in French.
Creperies are a good choice. They are inexpensive, typically French and crepes are made fresh each time, so they should be able to make a vegetarian filling for you even if it's not on the menu. Galettes are made of buckwheat and water (no egg or milk) and you can ask for a filling with just vegetables.
An 'assiette de crudités' is a salad plate of raw vegetables often served in traditional style restaurants, and usually has grated carrot, red cabbage, cucumber, potatoes, sweet corn, beetroot etc in vinaigrette. It is usually a starter or you can ask for it as a main course. Very nice with a side plate of chips!
If you order a quiche make sure they have not put ham into an innocent looking vegetable quiche! It is a good idea to ask if there is ham (jambon) in it. And beware, if you ask if there is any meat (viande) in it they do not always think of ham as being meat (!) so it's best to specify ham.