You could try plain soft, silken tofu. Throw it in your blender or food processor with some freshly squeezed lemon juice and a pinch of sea salt. Taste and adjust seasonings until it has that tang that you expect from yogurt. Also, I've seen yogurt made from soymilk. Inquire in health food sections of grocery stores or in health food stores. Last option, make a nut butter out of raw (or unroasted) cashews and add lemon juice and a pinch of sea salt. Again, taste until you get the right consistency. The downside of this option is fat content. Good luck!