Combine 1/4 cup rice milk and arrowroot and set aside.  Combine remaining rice milk, oats, brown sugar, agave, and salt.  Bring to a boil, stirring occasionally.  Once boiling, reduce heat to medium and cook for 5 minutes, stirring occasionally.  Turn off the heat.  Stir in the arrowroot mixture until slightly thickened.  Stir in the vanilla and refrigerate several hours until cold.  Pour into icecream maker and prepare according to manufacturer's directions (I doubled the recipe and mine took about 20 minutes).