Falafel in the Marais or an omelette from a tabac are good fallback options.
It can often be a bit of a trek for veggie food in Paris. Le Potager du Marais is a good start, if you're on the right bank Passage Brady has some OK Indian places which are very vegetarian-friendly. Lots of veggie places will advertise as 'Bio' so keep an eye out for that on signs.
If you know where you're staying I can think of more places to go, I've been in Paris quite a bit this year. Happy Cow has a listing (http://www.happycow.net/europe/france/paris/
index.html) but lots of places on that site are canteeny and open odd hours so it's not the most reliable if you're at a conference during the day.
