I studied French for several years, and the best advice I got from native french people, Mauritians, and travellers alike is to expect them to be rude to you. You could speak no French, or perfect French, but if it's noticable that you're not a native French speaker, they'll more than likely be rude to you.
That aside, don't let it deter you, France is a beautiful country.
I can understand the confusion with your host sister, as 'I am vegan' is 'Je suis végétalien' and 'I am vegetarian' is 'Je suis végétarien' ... It could be useful to say 'Je ne mangez pas d'œufs ou de produits laitiers' which is 'I don't eat eggs or dairy products' which could be tacked on the end of saying 'je suis végétalien' for clarity
ETA - buy a spencer and long johns, and layer it up for the cold weather. Layers are the key because you can strip off when you get inside