Paris has not progressed as far on the vegetarian food front as London has. In the Marais, it has actually gone backwards with many vegetarian restaurants formerly in the Marais having disappeared.  Standard Parisian brasseries and cafes have virtually nothing edible on the menu.  Even the rare Asian restaurant that purports to have a "vegetarian" part of the menu -- like Baan Lek, at 7 rue de Jarente -- puts pieces of chicken in their "vegetarian" tofu soup.  Among the formerly recommended restaurants in the Marais that had at least a few vegetarian options but are now gone are Fee Mini The, 25 Boulevard du Temple, Rose Bakery, 30 Rue Debelleyme, Les Enfants Gates, 43 Rue Des Francs-Bourgeois, and Le Marais, 54 Rue Ste-Croix-de-la-Bretonnerie.  Others, like Pain, Vin et Fromages, 3 Rue Geoffroy-l'Angevin, and Le Passage Oblige, 29 Rue du Bourg Tibourg, are closed for large parts of August when many foreign tourists are visiting.

 The recommended falafel shops on Rue de Rosiers are almost entirely meat restaurants, so if falafel is not to your liking, you are out of luck.  The best bet for a visiting vegetarian may be to rent an apartment where you can cook your own dinner, and eat your big meal at lunch at a tea room.  The best in the Marais is Le Loir dans la Theiere, 3 rue des Rosiers.  Their eggplant tart, carmelized onion tart, and apricot tarts are absolutely to die for.  Also good is L'Ebouillante, 6 Rue des Barres, which has most of its tables outdoors on a pleasant terrace, but has fewer vegetarian options.

If you do not mind a fairly long walk to the second arrondisement, A Priori Tea room, 35-37 Galerie Vivienne (or take the metro to Bourse), has a changing menu but you will often find four out of the five main course dishes to be vegetarian, and the food is gourmet.  There is a lovely covered passage where most of the tables are located.  The only fully vegetarian restaurant in the Marais where you can have dinner is Le Potager du Marais, 22 Rue Rambuteau, which is the continuation west of Rue Francs-Bourgeois.  The restaurant has two separate rooms and some space on the sidewalk for tables.  There is a set menu with four or five main courses listed (including two versions of a quinoa burger), some salads, and a soup, and there are daily specials.  The seitan bourguignon is good and is something you would not typically find in the U.S., and the French onion soup is truly delicious.  A recent special was goulash with vegetables and tofu, also good, although the flavor was very like the bourguignon. 

A second good choice is Le Passage Oblige, 29 Rue du Bourg Tibourg, which is mostly a non-vegetarian place but has several vegetarian options that are very well done.  If you don't mind a bit of a walk to the other side of Notre Dame, you can try Le Grenier de Notre Dame, 18 Rue de la Bucherie, which is fully vegetarian, but the food is stuck 20 years in the past, including the vegetable mush with a strong taste of lemon that was supposed to be pate, and the unidentifiable and strange-tasting vegetable balls that on a recent visit were the vegetable plate of the day.  If you must go, try the escalloped seitan, which was the only thing tried that was actually tasty.