Bring sugar, vanilla bean seeds and pod, rhubarb, and water to a boil in a medium saucepan, stirring to dissolve sugar. Remove from heat, and add raspberries and 1 to 2 tablespoons sugar if desired, depending on sweetness of the fruit. Let cool for 30 minutes.
Gently mash fruit with a ladle. Strain through a coarse sieve into a large bowl; discard solids. Chill soup at least 3 hours or up to overnight. Garnish each serving with a scoop of vanilla ice cream and raspberries if desired.