Add water, rhubarb, sugar, lemon zest, and vanilla to a saucepan and bring to a boil. Reduce heat, cover, and simmer for 10 minutes. Stir in 1 cup of strawberries and bring to a boil again. Let the mixture cool. Strain through a sieve. Stir in the remaining cup of sliced strawberries and a cup of lemon juice. Pour over ice and enjoy.