If I could recommend only one veg restaurant in Paris, this would be it. When I lived in Villennes (half hour west of Paris), I went here nearly every weekend -- for my stomach and my soul. Marie-Cécile, the incredibly warm proprietress, is the only constant person there (though there is sometimes someone else helping out in addition to or instead of her). She speaks great English, but won't force you to if you'd rather speak French (immediately wins points from me, esp. in Paris). The whole experience is rather like eating in someone's kitchen, in the best possible way.
The place is a hole in the wall, bursting with character but with very limited seating, and will often change hours depending on the season or lack of food or lack of people. ALWAYS call before going if you are intent on eating there -- it's sometimes closed when it's scheduled to be open (often closed on Mondays and Sunday night), and sometimes open when it's scheduled to be closed (the hours on the door are generally not-quite-correct). Weekend nights (and sometimes weekday nights) can get crowded, but there is a marvelous ambiance and conviviality that's often at its best at these times. Call ahead and you'll rarely have a problem.
The menu is strangely laid out; do your best and/or ask questions. There are sometimes salads (never all the ones listed), often hummus, and always a vegan plate and a vegetarian plate. (This is the extent of the non-dessert food, although sometimes there is soup or homemade seitan or some other special; they are a small place and prefer to do fewer things better.) The two plates are largely the same, with a few differences. They consist of a bunch of little piles of different things -- some raw, some cooked, maybe a vegetable tart -- and something on bread (goat cheese or vegetable pâté), though the exact composition depends on the season and whims. It is some of the most satisfying and nourishing food I've ever had (I LOVE having lots of different things, for one), and she puts some kind of magic salt on it that is stellar.
I highly recommend the apple crumble, too, especially if it's fresh or warmed up -- it is vegan, it does not taste like punishment for eating/being vegan, and it's fantastic. Other desserts make appearances, but the crumble is the most constant.