Halve vanilla bean and scrape seeds into a 2 quart heavy saucepan, then add pod and remaining ingredients. Bring to a boil, stirring until sugar is dissolved., then boil until syrupy and reduced to about 1 1/3 cup, about 20 minutes. Cool to warm and discard vanilla bean pod.
Syrup can be made 3 days ahead and chilled, covered. Reheat over low heat.