1. In a small saucepan combine the rhubarb, ½ cup strawberries, sugar and water.
2. Bring to a boil, reduce the heat and simmer until the rhubarb is very soft, about 15 minutes.
3. Strain the mixture into a bowl using a fine mesh sieve, pressing to get all the juice.
4. Add the vanilla to the strained syrup. (Save the rhubarb-strawberry solids to use on pancakes or toast.)
5. To assemble the sodas, pour 3–4 tablespoons of syrup into each glass, fill the glass 2/3 of the way with soda water and stir.
6. Drop in 1 scoop of vanilla ice cream followed by ¼ cup strawberries, then another scoop of ice cream.
7. Break out the bendy straws.

Note: Any leftover syrup can be stored, tightly covered, in the refrigerator for up to 1 week.