Wash and dry all produce. Halve cucumber lengthwise, then slice crosswise into thin half-moons. Toss cucumber with 4 tsp vinegar (we sent more) and 1/2 tsp sugar in a small bowl. Season with salt and pepper and set aside. Pat chicken dry with a paper towel.

Heat a drizzle of oil in a large pan over medium-high heat. Add cabbage. Season with salt and pepper. Cook, tossing occasionally, until softened, 3-4 minutes. Remove pan from heat and stir in remaining vinegar. Transfer contents of pan to a medium bowl.

Heat a large drizzle of oil in same pan over medium-high heat. Season chicken with salt and pepper, then add to pan. Cook, tossing occasionally, until browned and cooked through, 3-5 minutes.

Reduce heat under pan to medium, then pour in bulgogi sauce. Bring to a simmer and let bubble until sauce has reduced slightly, about 2 minutes, while tossing chicken to coat. Stir in cabbage mixture, then remove pan from heat.

In another small bowl, stir together sour cream, 1 tsp water, and sriracha to taste. (TIP: Add more water as needed to achieve a drizzling consistency.) Wrap tortillas in a damp paper towel and microwave on high until warm, about 30 seconds.

Divide chicken and cabbage mixture between tortillas, then top with cucumber. Drizzle crema over each taco.