White wine substitute in potato leek soup
Why substitute anything? Cooking is all about tasting, if you like how the soup tastes without the wine then there's no need to add anything to replace it. The white wine supplies acidity, which you may not actually want if you've already added sour cream. It also supplies fruitiness which again may or may not be to your taste. If you want more acidity then squeeze some lemon juice into it, if you want fruitiness add maybe 1/8 cup of white grape juice. Add a bit at a time, tasting as you go to make sure you don't overdo it. 

If you decide you like the soup without the wine then don't add anything, your recipe won't miss the 1/4 cup of fluid.