In a medium size bowl mix together the vinegar, orange juice, red pepper flakes, honey, and then add the rosemary. Set aside.
Pat salmon fillets dry with paper toweling and season them with salt and pepper.
Place oil in a large nonstick skillet over medium-high heat. Cook skin side up until nicely browned. Flip the salmon fillets to skin side down and cook until skin is crispy and salmon is done.
Remove salmon to a serving platter and tent with aluminum foil to keep warm. Lower skillet heat to simmer, wipe skillet clean with paper toweling, and gently pour the balsamic mixture into the skillet. Cook until mixture is syrupy. Remove rosemary sprig and stir in the butter until melted. Pour the sauce over the fish and serve hot.