Huh... that should be pretty safe, then. I have no idea where the off flavor would come from.

Try adding something sweet and see if that helps (my sweetener of choice for soups like this is usually ketchup (it sounds odd, but true), but I don't know how the vinegar would affect the sour taste). Maybe just 1/4 teaspoon of plain sugar so there aren't any other flavors to complicate things.

Also, try a pinch of salt. There should be plenty from a bag mix of vegetables, but a small amount of salt can do wonders.