In bowl of food processor, finely chop grapes and sugar using one-second pulses. Transfer to deep, medium skillet or wide saucepan; stir in lemon peel, salt and water. Bring to boil over medium-high heat. Cover pan with lid, reduce heat to medium-low and simmer 18 minutes until fruit is tender.

In small bowl, stir lemon juice and cornstarch then stir into grape mixture with butter; simmer 6 minutes, or until desired consistency. Stir in vinegar, if desired. Serve warm or cool.

Cooled jam can be stored in covered glass jar in refrigerator up to 2 weeks.