I had two good vegan meals recently...one at Golden Era, the other at Gracias Madre.
Gracias Madre isn't downtown, but it's very easily accesible via bus, taxi, or BART from downtown. Depending on where you're coming from, might actually be easier to get to than certain places in the Tenderloin.
Millenium - 580 Geary Street (Tenderloin)
Golden Era Vegetarian - 572 O'Farrell (Tenderloin)
Loving Hut - 845 Market (Westfield)
Enjoy Vegetarian - 839 Kearny Street (Financial)
Plant Cafe - 101 California (Financial)
Urban Picnic - 125 Kearny St (Financial)
Lucky Creation - 854 Washington (Chinatown)