Cut peels from grapefruit and orange. Cut pith from peels, and slice into very thin strips. Slice fruit along membranes to release segments into a bowl. Squeeze juice from membranes into bowl; discard membranes. Pour cup juice through a fine sieve into a small bowl.
Bring a medium saucepan of water to a boil. Cook citrus peels for 1 minute; drain. Bring cup water, the sugar, and vanilla seeds and pod to a boil in a clean medium saucepan over medium-high heat, stirring until sugar dissolves. Reduce heat to low. Add reserved juice and peels and the salt; cook for 2 minutes. Let cool completely.
Discard vanilla pod. Toss syrup with reserved fruit.