Place saffron threads in a small skillet; cook over low, swirling skillet constantly, until threads are dry and toasted, about 1 minute and 30 seconds. Crumble toasted saffron into a small bowl; add 2 tablespoons hot water, and stir to combine. Let steep 10 minutes. Bring a large pot filled with salted water to a boil over high. Add asparagus; cook, stirring often, until vibrant green, about 1 minute. Using a spider, remove asparagus, and plunge into a bowl filled with ice water; let stand 3 minutes. Drain and set aside. Add spaghetti to boiling water in pot; cook, stirring occasionally, until just al dente (about 2 minutes less than cook time specified in package directions). Drain and reserve 1 cup pasta cooking liquid. While spaghetti cooks, heat oil in a large, deep skillet over medium until shimmering. Add shallot, garlic, and crushed red pepper; cook, stirring constantly, until slightly softened, 1 to 2 minutes. Add asparagus, prawns, and saffron-water mixture; cook, gently turning prawns occasionally, until light pink in spots, about 2 minutes. Add vermouth; bring to a boil. Transfer prawns to a plate using a slotted spoon; continue boiling mixture in skillet, undisturbed, until liquid is slightly reduced and alcohol flavor cooks off, 3 to 4 minutes. Add cooked spaghetti to asparagus mixture in skillet; toss to coat using tongs. Return prawns to skillet; add butter, parsley, tarragon, and chives. Increase heat to medium-high; cook, stirring and swirling skillet constantly, adding reserved cooking liquid as needed (1/4 cup at a time) until a creamy sauce forms and coats noodles, about 3 minutes. Season with salt to taste. Serve immediately.