The best cake I've had in San Francisco was from Sugar Plum Vegan. It was a beautiful vegan cake that was also free of gluten and soy, yet it still tasted delicious (even to non-vegan friends)! They have a location in Sacramento and sell to stores (like Rainbow Grocery) in SF. Click here to see their wedding and party menus.
Mocha Brownie from Sugar Plum
Here are places other people have recommended:
* Vegan Wheekers described her cake from Millennium: "pastry chef, Anne, is a pro! I ordered a week before the party, only spoke to her on phone w/ general specs and she created my dream cake using real flowers for decoration. It was a small two tier cake (each tier diff flavor). Loved it! Mocha & hazelnut w/ white chocolate icing"
If you're looking for more wedding vendors, be sure to visit Rose Pedal Vegan Weddings (where yours truly is listed as a photographer).