In small bowl, combine fish sauce and brown sugar, stirring to dissolve sugar. Transfer 1 tablespoon mixture to large bowl and set aside.  Heat oil in large skillet on medium. Season steak with 1/2 teaspoon each salt and pepper and cook, brushing with fish sauce mixture frequently and turning occasionally, until browned on all sides and at desired doneness, 12 minutes total for medium-rare. Transfer to cutting board and let rest at least 5 minutes before slicing.  To large bowl with remaining fish sauce mixture, add rice vinegar, chile and 1/4 teaspoon salt, stirring to dissolve salt. Toss with carrots, radishes and cucumber until evenly coated, then fold in scallions and mint. Serve with steak, sprinkled with peanuts.