I just went on a trip to Italy and France with my school on March Break. The tour company asked about food restrictions ahead of time, so they could make arrangements for breakfast and dinner. I am not really vegan (yet) but I avoid dairy and eggs wherever possible, so I did sometimes have a hard time finding lunch and snacks within the limited time frame. The veg. food was awesome in Italy, where it's just normal to use fresh vegetables and olive oil.
In France, the food options were not so impressive, with the exception of the bakeries near our hotel. But then again, we only stayed in Paris for 2.5 days and we were going on a budget, so of course we couldn't afford to eat at the better restaurants in an extremely expensive city like Paris. You'll probably have more time to look for vegan options, since you'll be there a bit longer. But anyway, one night I walked into the restaurant and saw the cook serving both chicken and vegetables with the same scoop, so I just couldn't eat that part. And everyone else in our group got barely-cooked meat of some sort for that meal, so make sure you tell them you're vegan!! Croissants are cooked with butter, there is Milka chocolate in all the stores, and even the pizza at the restaurant was cheese-filled. I am sure there is lots of vegan food available in Paris, but it might not be easily accessible on a tight schedule. You might need to make a stop at a store if your tour company doesn't provide you with meals and such. Food is pretty cheap at the markets and supermarkets, so if you have time to find one it would be worth it. The touristy areas are typically where it's tough to find veg. food, I think. Like, I saw fast food restaurants and high-end on and near the Champs-Élysées, but near our hotel we had stores with actual food and bakeries so we could get something decent for lunch.
If you look up where your hotel is, you might be able to find a store nearby before you go! We were lucky and got a really nice hotel near a métro station in the business area of Paris, and there were lots of stores nearby.
Anyway, good luck! The trip will be amazing, I'm sure.
Take a million pictures... and if you have a chance to go to Versailles, definitely go! It was absolutely amazing. One of my favourite things about Paris was definitely the métropolitain, even though it was scary at first. Yes, I loooooooved the métro SO much!!! I miss Italy and France so much now.
OOPs, just noticed that you already told them you're vegan.
I don't know what tour you're going with, but our company took quite good care of us and the trip was amazing... (Oh, and I'd be remiss if I didn't mention our tour director, who was totally awesome, and all the girls fell in love with him by the end of the trip... hehe)