In a small bowl, mix the yogurt, mayonnaise, chives, capers, caper juice, and vinegar together. Taste, and add salt according to how you are going to use the sauce (less if a dip for salty chips or on top of well-salted rösti, a bit more if dressing potato salad or a baked potato). It can be served immediately, but is best if covered and refrigerated for at least an hour ahead of time.