Is cow eyeball vegetarian? Anyhow, the best vegetarian tacos I've ever had (maybe the only one's I've ever liked) were at Maxwell street market. Manalo's and/or Rubi's (can't remember which, or if it's both) serve Huitlacoche (the corn mushroom spoken about up-thread), as well as flor de calabaza (pumpkin blossom). They also have regular mushroom tacos, though i don't recall what I thought of those. Their hand made tortillas with cheese, coupled with one of those fillings will not disappoint you. They are huge, and filling. Some days when I eat there, 2 is almost too much for me, and I'm typically a 3 to 4 taco kind of guy. There will be numerous other options. Part of the fun of the market is doing some of your own exploring, splitting a taco or two at a number of places to try to fit in more than just a couple stops along the way.
Part of the secret of a success in life is to eat what you like and let the food fight it out inside.
-Mark Twain