In a small saucepan, heat sugar with 1/2 cup water, the vanilla-bean pod and seeds, and spices over high heat until sugar dissolves. Remove from heat, steep 30 minutes, then strain.
Meanwhile, cut peel (including all white pith on outside of fruit) from grapefruit, following curve of fruit. Slice grapefruit crosswise into rounds about 1/4 in. thick, removing any seeds. Arrange on a large platter. Repeat with oranges.
Drizzle citrus with about 1/3 cup spiced syrup. Scatter lime zest on top. Serve with a little more syrup if you like.