In a medium bowl, combine the chopped rhubarb, flour, sugar, baking powder, baking soda, salt, and cinnamon. Mix well to ensure the rhubarb is coated with the dry ingredients.
In another bowl, whisk together the milk, egg, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay.
Heat the vegetable oil in a large skillet over medium heat. Once the oil is hot, drop spoonfuls of the batter into the skillet, making sure not to overcrowd the pan.
Fry the fritters for about 2-3 minutes on each side, or until they are golden brown. Use a slotted spoon to transfer them to a paper towel-lined plate to drain excess oil.
Dust the warm fritters with powdered sugar before serving.