In a saucepan, heat 1/2 cup water, the vinegar, sugar and 3/4 tsp. salt over medium heat until the sugar dissolves, 2 to 3 minutes. In a bowl, combine the vegetables. Pour the hot liquid on top and toss well; let sit for 10 minutes.
In a skillet, brown the beef over medium-high heat, about 8 minutes. Add the coriander; cook, stirring, for 2 minutes. Season with salt and pepper. Transfer to a plate.
Add the beans and 1/2 cup water to the skillet. Cook over medium heat, scraping up browned bits, until thickened, 3 to 5 minutes. Remove from heat; add sour cream. Smash the mixture with a potato masher; season with salt.
Spread the beans onto the tortillas. Top with the beef, pickled vegetables and cheese.
