You can barely throw a stone in that part of San Francisco without hitting an Indian restaurant, which is probably the lowest difficulty cuisine to eat vegetarian. If you want to stuff yourself for essentially nothing, I recommend the local chain Naan and Curry which is a passable Indian buffet with great desserts. But for an unforgettable experience, walk the extra bit to Shalimar. The entire restaurant is essentially a giant tandoor, so if you don't mind coming home smelling like Indian seasonings and smoke, you can enjoy mouthwatering delights.
There's also a Taqueria Cancun not far from the St. Francis on Market. Every year since the category was started in a local paper, they've been voted the best vegetarian burrito in SF, but beyond that, it's a great example of the Mission style of burritos. It's also $6 for about 2 pounds worth of burrito. You won't be going home hungry or break the bank.