As a vegan resident of Yokohama I eat Indian a few nights most weeks, and Sitar is my favourite. The owner, Kumar, who is virtually always there, understands well what vegans can and can't eat, and I have never had problems at all, in going there at least once a week for almost a year. The food is always fresh and delicious, seating is comfortable, decor is pleasant and service is always friendly and efficient. The papadoms and mango and sweet chutneys served upon arrival are a nice touch. For the overall balance of food, price, comfort, service and location, Sitar wins as the best Indian restaurant in Yokohama, and as far as I can find, the best meal out in the city.
To get there, take exit 2 (for Chinatown, not Motomachi) and walk North. At the third set of traffic lights (including the ones straight after the exit) turn left. Sitar is at the end of the street, on the left. It is well signposted and stairs lead down to the restaurant.
Cons: only four vegan mains