Apple cider vinegar will work, although has a certain flavor that tends to creep into the Concord grape flavor, so you'll have to decide if you want apple flavor in your shrub, or to keep it purely grape. I generally use white vinegar. There's a recipe for a cocktail after the instructions which uses the shrub, but it can also be enjoyed in a glass with sparkling water added and an orange or lemon twist, if you'd like.