Unforntunately, Midtown is something of a vegan deadzone. I don't know of any strictly vegan restaurants north of Franchia (Korean/Asian fusion at 34th and Park) until you get up to around 79th street (Blossom on the UWS and Candle Cafe, Candle 79, Gobo and V-Note on the UES).
One option is the Dig Inn (formerly 'Pump Energy Foods') on Broadway, between 39th and 40th. It's not even a vegetarian restaurant but they usually have a lot of vegan options. It's one of these places where you start with a base (brown rice, cooked greens, salad greens or a wrap) and add protein (tofu is one of the options) then choose from a number of vegetables and other things like bean chili, lentil soup, and then a sauce or dressing (romesco, hummus, basalmic viniagrette). They have a wicked good peanut butter-banana shake that can be made vegan if you ask them to leave out the whey powder, and use soy milk.
There is a Chipotle on 7th Ave at 37th, and a Subway on 44th between 6th and 7th, if you get desperate.