Place steaks in a large resealable plastic bag. In a bowl, whisk together 1/4 cup of the vinegar, the soy sauce, canola oil, 2 tbsp of the sugar, 1 tbsp of the sriracha and 1 tsp of the sesame oil. Stir in onion, garlic and ginger. Pour into bag, making sure steak is covered. Marinate in refrigerator 1 hour.
In a separate bowl, whisk together remaining 1 tbsp each vinegar and sugar, and remaining 1 tsp each sriracha and sesame oil. Stir in salt and cucumber. Cover bowl with plastic wrap and refrigerate 30 minutes.
Heat a large sauté pan to medium-high heat. Remove steak from marinade (save marinade) and pat dry with paper towels. Cook 2 to 3 minutes per side, until medium-rare. Set aside to rest 5 minutes. Meanwhile, add marinade to pan. Bring to a boil and cook 3 minutes, until sauce has thickened.
Thinly slice steak; toss in pan with sauce. Serve on tortillas with pickled cucumbers and, if desired, sliced radishes, sesame seeds and additional sriracha.