WHISK 1 tbsp soy with vinegar and sriracha in a small bowl. Set aside.
WHISK egg with water, 2 tbsp oil and remaining 1 tbsp soy in a large bowl. Whisk in flour, baking powder and salt until smooth. Stir in cabbage and shrimp. Reserve 2 tbsp green onions. Stir in remaining green onions.
HEAT an extra-large non-stick frying pan over medium-high. Add 1 tbsp oil. Scoop heaping 1/3-cup portions of batter into pan, pressing to flatten slightly. Reduce heat to medium. Cook until the bottom is set and golden, 3 to 4 min, then flip. Cook until edges start to crisp, 3 more min. Transfer to a plate. Repeat with remaining oil and batter. Sprinkle with remaining green onions. Serve with vinegar sauce.