Rinse rice thoroughly until water runs clear. Place in a large bowl with 4 cups cold water and a cinnamon stick. Soak for 12 hours.
In a saucepan, combine almond milk and sugar. Simmer over medium heat for 20-30 minutes until reduced by half. Cool to warm.
Drain the soaked rice and transfer rice and cinnamon sticks to a blender. Add 2 cups of reserved water. Blend for 3-5 minutes until smooth.
Strain the blender contents through a fine-mesh strainer, cheesecloth, or nut milk bag to remove lumps and bitterness.
In a large pitcher, combine the strained rice water, reduced almond-milk mixture, and 1 ½ teaspoons vanilla extract. Stir well and refrigerate until chilled.
Serve chilled in glasses over ice, sprinkling a pinch of ground cinnamon on top if desired.