Set a large soup pot on the stove over medium-high heat.

Add the oil to the pot and heat it until it looks loose and shiny, about 30 seconds.

Add the sliced shiitake mushrooms to the pot. Cook, stirring every minute, until they shrink and start to brown at the edges, 4 to 5 minutes.

Add the scallion whites to the pot. Cook for 30 seconds, stirring, until they smell fragrant.

Add the sliced cabbage to the pot. Cook, stirring often, until it starts to soften but still looks bright, about 2 minutes.

Pour the vegetable broth into the pot and raise the heat to high.

Stir the tamari into the broth until it is evenly mixed.

Stir in 2 tablespoons of the rice vinegar, saving the last tablespoon for the end.

Add the tofu cubes to the pot. Let the broth come to a full boil.

Put the rice noodles into the boiling broth. Press them down with a spoon so they are mostly covered by liquid.

Cook the noodles in the pot, stirring once or twice so they do not clump together, until they are tender, 3 to 4 minutes.

Put the cornstarch in a small bowl. Add 2 tablespoons cold water and stir until the mixture is smooth.