Stir together the plum sauce, ketchup, soy sauce and pepper flakes in a small bowl and set aside.
Sprinkle the shrimp with salt and pepper. Heat the oil in a medium skillet over medium-high heat. Add the shrimp to the skillet and cook, stirring occasionally, until just cooked through, 2 to 3 minutes. Transfer to a plate.
Add the scallion whites, garlic and ginger to the skillet and cook, stirring constantly, until soft, about 1 minute. Add the vinegar and scrape up any brown bits that cling to the bottom of the skillet. Add the plum-ketchup sauce and bring to a simmer. Return the shrimp to the skillet along with the scallion greens and give the skillet a swirl to bring everything together. Divide among 4 plates and serve with white rice if using.
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