In a small bowl, whisk together miso paste, rice vinegar and 1/2 cup water until smooth. Stir in 2 of the sliced scallions. Cover and set aside until serving.
In a separate bowl, whisk together wasabi and soy sauce until smooth. Stir in remaining scallions, the shrimp, water chestnuts and ginger. Bring a large pot of lightly salted water to a boil. Place 1 wonton wrapper on a dry surface. Add 2 tsp filling to center and brush edges with egg whites. Bring together corners and press firmly to seal (see folding instructions on package). Repeat with remaining wrappers. (Cover wrappers and dumplings with damp towels while prepping.)
Add 12 dumplings to boiling water. Cook 3 minutes; dumplings will float to surface. Repeat 3 more times, until all dumplings are cooked. Serve hot with miso dipping sauce.