Mix the rice flour, arrowroot & 1/4 c cold water. Turn the mixture out onto a smooth surface & knead it to the consistency of a pie crust. Add more water if needed. Shape into 1 1/2" balls. Fill a large pot half full with water & bring to a boil. Drop the balls into the boiling water, four at a time. Boil for 4 to 5 minutes or until they float to the top. Cook the rest of the balls this way. In a large pot, heat the coconut milk, sugar, salt & vanilla over medium heat for 5 to 7 minutes. Add the rice balls & cook, stirring constantly, until the milk starts to boil. Serve hot.