To make the sake essence: In a heavy 2-quart saucepan, combine the sake, mirin, rice vinegar, ginger, garlic, and serrano. Bring to a boil over high heat, and reduce to about 1/2 cup, 10 to 15 minutes. Meanwhile, heat the oil in a skillet over medium-high heat. Saute the peppers, zucchini, and carrot until crisp-tender, about 2 minutes. Add the soy sauce and vegetables to the sake essence; set aside and keep warm.

To prepare the salmon: Sprinkle both sides of each salmon fillet with salt and pepper and drizzle with a little of the vegetable oil, using 1 tablespoon total for the four fillets. Completely immerse one round of rice paper in a bowl of warm water for a few seconds. Transfer to a work surface and let stand until pliable, about 30 seconds. Set a salmon fillet in the center of the softened rice paper. Fold the paper over one long side of the fillet, and then fold the paper in over the two short sides. Roll the fillet over until it's completely enclosed by the rice paper. Set it aside with the seam side down. Repeat with the remaining fillets.

Heat the remaining 3 tablespoons oil in a heavy 12-inch skillet over medium to medium-high heat until it's hot and shimmering but not smoking. Put the wrapped fillets in the skillet without touching and saute, turning to brown top and bottom, until the rice paper is golden brown, 2 to 3 minutes per side; don't overcook. (If the fillets are very thick, you can also brown the sides for 1 minute each, but most fillets will be cooked through--just barely opaque--if just the top and bottom are seared.) Transfer to paper towels and let drain.

Stir the chopped cilantro into the sake essence and spoon some onto each of four warm dinner plates. Put a salmon fillet in the center of each plate, top with a cilantro sprig, and serve at once.