In a small saucepan over medium heat, combine the sugar with ¾ cup water. Bring to a simmer and cook until the sugar is dissolved, about 5 minutes. Add the mango juice and stir until incorporated. Add the vanilla bean and pulp and simmer for 2 minutes to bring out the flavor. Add the lemon juice, taste, and add more lemon juice if needed. The sauce should be a pourable consistency; if it’s too thick, add a little water. Remove the vanilla bean before using.