Cafe Gratitude does a neat thing on Thanksgiving where you can get a free vegan meal. They always need people to volunteer to serve and wash dishes and stuff.
http://cafegratitude.com/events/all-upc ... sion-cafesThat's not really a soup kitchen, but it's vegan and is open to anyone.
You could also email the sf food not bombs and see if they're doing a Thanksgiving serving.