I use a vanilla mix (non-dairy) from Precision Foods. I will use some kind of nut milk with it instead of water (these days I have been using unsweetened cashew milk) and flavoring. I am not making soft serve but I am churning and freezing it like normal ice cream, but it would be soft serve if eaten immediately after churning or, I'm assuming, made in a soft serve machine. I also found if you want to keep it scoopable when frozen, adding vegetable glycerin does the trick.
My recipe is:
200 grams of mix
16 oz (liquid measure) of unsweetened nut milk (or soy or coconut)
1 tbs vegetable glycerin
approx 1/4 cup of flavoring if liquid or 4-5 tbs of powder like peanut butter powder or cacao.
Here's a recent batch of peanut butter banana with hand chopped dark chocolate. 4 tbs powdered peanut butter and 1/4 cup Torani banana flavor.