A BIPARTISAN COALITION of U.S. congressmen announced Friday they will nominate African National Congress Deputy President Nelson Mandela for the 1991 Nobel Peace Prize. . . . VIETNAM'S PREMIER war hero, Gen. Vo Nguyen Giap, boosted by improving relations with China, is favored to be Vietnam's next president in a hotly contested leadership shakeup expected next year. . . . THE NEW ORANGE crop, led by a robust recovery in Florida from last winter's devastating freeze, will fill 231 million boxes, 24 percent more than last season, the government says.