Cook gently, covered, for 5 to 6 minutes, or until rhubarb is tender but still retains its shape. Cool rhubarb to room temperature before proceeding, or chill until needed, up to 2 days. Bring to a gentle simmer. Have ready fine-mesh sieve set over bowl to strain cooked custard. Pour cream into 2-quart saucepan with heavy bottom.
Scrape seeds out of vanilla bean.
Add seeds and hull to cream. Place pot over medium heat. Bring just to a boil. Pour in granulated sugar, whisking constantly. Place mixture over pot with barely simmering water.