Season the fish with salt and pepper. Put the flour, eggs and panko in separate shallow dishes. Dredge the fish in the flour, then dip in the egg, letting the excess drip off, then coat in the panko. Transfer to a plate.
Make the dressing: Stir the mayonnaise, buttermilk, lemon juice, scallions and a pinch of salt in a small bowl. Refrigerate until ready to serve.
Trim the ends of the zucchini and yellow squash and halve crosswise. Shave lengthwise into thin ribbons with a mandoline or vegetable peeler. Toss in a large bowl with the vinegar and vegetable oil; season with salt and pepper.
Heat 1/2 inch vegetable oil in a large skillet over medium-high heat. Add the fish and fry, rotating the skillet occasionally, until the fish is browned around the edges, 3 to 4 minutes. Carefully flip and fry until browned on the other side, 2 to 3 minutes. Transfer to a rack set on a baking sheet and season with salt.
Divide the fish and vegetables among plates. Top the fish with the dressing; sprinkle with the scallion greens. Serve with lemon wedges.