You'll have lots of choices in Paris. As Maria suggested, I use HappyCow.net but often I'll just stroll wherever I am and read menus. I don't want to travel across town to a "vegan" restaurant when I'm hungry and a restaurant nearby will work.
Do get familiar with French words for menu items so you can easily rule out beef/pork/veal/lamb/various seafood/milk products. Rick has a small French phrase book and dictionary that is handy. I've got a more extensive food phrase book but it's too complex so I just use his phrase book if there is something I don't recognize.
When I am in Paris I do love falafel. I don't have much available locally that would even pass for falafel so I get myself over to the Marais and eat at my favorite falafel place Mi-va-mi. Across the pedestrian street is a maybe more well known place L'As du Fallafel but I like the food better at Mi-va-mi. It was recommended to me by another RS forum member, Donna. It is usually inexpensive - a falafel pita and drink is maybe 10E.
Another falafel place without much of a sit-down area is Maoz. I like the one on Rue Saint-Andre-des-Arts in the Saint-Germain-des-Pres area. It is even less expensive - maybe 8E for a falafel pita and a drink.
Some boulangeries sell pre-made baguette sandwiches which are inexpensive. Many offer just meat or have butter on them but there is one near Notre Dame that does a very nice vegan one - just veggies and no butter or mayo. The place is called Hure and it's on Rue d'Arcole.
There is also a gelato shop that has vegan sorbetto. It's called Amorino and has a number of shops thru Paris. They generally have their vegan selections labeled but staff is usually very well-informed.
I've traveled to Mont-Saint-Michel but was with a Rick Steves tour group. The guide was able to get me a vegan meal for our group meal there but otherwise I think it might be difficult. Most of the restaurants on the island are pretty touristy and serve a lot of seafood or egg-based dishes.
Normandy and the Loire Valley will be easier altho you may have to fall back on Italian food or salads.
Grocery stores everywhere (except maybe MSM) will have prepared salads. I often go in to look for small tubs of hummus and get bread to go with. Do read the labels as sometimes the hummus has some kind of cream added.
Have a wonderful time!