Zest grapefruit, then section the fruit, cutting it away from the peel and inner membranes. You should have 5 teaspoons zest and 3/4 cup grapefruit sections. Puree zest, sections, and any juices with sugar and vinegar. Add tea bag. Chill, covered, 4 hours to 2 days. Remove tea bag and strain.
In a cocktail shaker half-filled with ice, combine 2 fluid ounces shrub and vodka. Shake until very cold. Strain into martini glass or an ice-filled old-fashioned glass. Garnish with a grapefruit wedge.
