I may be totally mistaken but I don't know of any so-called "medium sized" churches in SF with 500-1000 members; that sounds pretty large sized to me...
However, one church that appears to be vibrant is St. Bede's Episcopal Church. They allow their building to be shared (probably through a rental arrangement) with HaMakom, a progressive Jewish congregation. But more than that, the two seem to have also forged a spiritual sharing - working together on causes benefiting the overall community.
Now that's the kind of vibrancy I like to see!