Add vinegar to a microwave-safe bowl; I use a 4-cup glass bowl or measuring glass. Microwave until hot, about 1 minute. Add diced cucumbers to the hot vinegar, and stir until well-coated. Set aside at room temperature.
Add mayonnaise and sriracha to a second bowl, stirring until very smooth. Add soy sauce, sweetener, sesame oil, and garlic to a third bowl, stirring until well-mixed and aromatic. Set them aside.
Add beef to a cold skillet; I use a 10-inch nonstick pan. Over high heat, cook until its juicies have evaporated and it begins to fry in its own fat, 8 to 10 minutes, crumbling and stirring frequently. You can tell it’s frying in its own fat when there’s darker browning on parts of the beef touching the pan.
Add the soy sauce mixture to the skillet and cook until it’s nearly evaporated, about 2 minutes, stirring frequently so that the beef is well-coated with sauce. Turn off the heat.
Divide the beef mixture among the lettuce leaves, prioritizing rounder cup-shaped ones. Strain out the pickled cucumbers and distribute them over the beef. Spoon the sriracha mayo on top. Serve immediately. If you have leftovers, store the lettuce separate from the other ingredients.