By unsweetened I assume you mean the kind you could use as sour cream and not the kind with fruit and sugar? I am in the UK and buy Alpro Soya yogurt on occation but make my own mostly. There has to be some sweetener for the cultures to react to and make yogurt. I use soy milk sweetened with apple juice (it comes that way) to make my yogurt but if they are out and I buy unsweetened soya milk I add 2 TB agave or maple syrup to give some sugar for the cultures to feed from. It still comes out tart and can be used for sour cream (with a bit of lemon juice added) not sweet like the fruit and sugar ones. If I want that I add some jam to what I've made.
You can google recipes to make soy yogurt. basically I sterilize everything with boiling water, fill my 1 litre jar with boiling water and let it heat up then heat the soya milk until it's 180 degrees and then take it off the heat, put in in a sink of cool water to drop the temp and then when it is 120 degrees i stir in a few TB of storebought yogurt as a starter. (I use the soy yogurt as a starter but have made the starter with cows yogurt when it was all I had) then pour the boiling water from the hot jar, dry with a paper towel and then pour in the hot milk and wrap in 2 tea cosys and put in my airing cupboard overnight next to my hot water heater. The next morning, put in the fridge before I go to work. Come home and it is ready. It is thinner than commercial yogurt so if you want it thicker--tie it in a tea towel hang it on the tap over the sink and let the whey drain out. Then it will thicken up.
edit: save back 2 TB of what you make to be your starter for the next batch.
