Slice cucumbers thin as possible and put in a bowl. Add the vinegar, sugar, salt, and crushed red pepper; toss to combine. Set aside until ready to use. (The longer the better).
For the Korean Beef:
Spray crock pot with non-stick cooking spray and add the beef. In a small bowl, mix remaining ingredients and pour over the meat. Cook on low heat for approx 8 hours stirring once or twice. Take meat out of crockpot and shred, and put back into the sauce.
For the tacos:
Warm tortillas in the microwave, add shredded beef, cucumber slaw, bean sprouts, and cilanto. If you are feeling brave, top with sriracha.