Prepare Oven and Pan: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
Mix Dry Ingredients for Batter: In a large mixing bowl, sift together the flour, granulated sugar, baking powder, baking soda, and salt.
Combine Wet Ingredients: In another bowl, whisk together the vegetable oil, almond milk, vanilla extract, and apple cider vinegar.
Prepare Cake Batter: Add the wet ingredients to the dry ingredients, mixing until just combined.
Gently fold in the chopped rhubarb and crystallized ginger until evenly distributed throughout the batter.
Pour Batter into Pan: Transfer the cake batter to the prepared cake pan, smoothing the top with a spatula.
Make the Crumble Topping: In a small bowl, mix the rolled oats, brown sugar, and cinnamon.
Add the chilled cubed vegan butter and rub it into the oat mixture using your fingertips until the mixture resembles coarse crumbs.
Add Topping and Bake: Sprinkle the crumble topping evenly over the cake batter.
Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out clean.
Cool and Serve: Allow the cake to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.
Serve as is or with a dollop of vegan cream or ice cream.