In a sauce pan, combine rhubarb, strawberries, water, agave, mint leaves, lime juice, and ground vanilla.
Bring to boil and simmer for 25 minutes.
Place a fine-meshed sieve over a bowl or pan and strain the lemonade. (Don't throw away the pulp; it's great with yoghurt).
Fill the lemonade into a large pitcher and let it cool in the fridge for about an hour.
Serve with fresh mint, lemon, and lots of ice.