Marinate the beef: In a small bowl, whisk the soy sauce, vinegar, sesame oil, sugar, chili sauce, garlic, and scallions. Put the beef in a large resealable plastic bag or shallow bowl and pour the marinade on top. Seal or cover and refrigerate for at least 2 hours and up to overnight.
Make the slaw: In a small bowl, whisk the fish sauce, soy sauce, vinegar, sesame oil, sugar, chili sauce, garlic, and ginger.
In a large bowl, toss the cabbage, carrots, radishes, scallions, and bell peppers. Pour the dressing on top and toss to coat. Cover and refrigerate for 30 minutes.
Grill the beef: Preheat your gas grill to high. Remove the steak from the marinade and discard the liquid. Once hot, grill the steak for 5 minutes per side. Transfer the steak to a cutting board and let rest for 15 minutes before slicing thinly across the grain.
Lay corn tortillas on a work surface, divide the steak among them, and pile a spoonful of zesty ginger slaw on top. Serve immediately.