NEARLY TWO DOZEN students took over the office of Massachusetts Institute of Technology President Charles Vest Thursday, demanding a campuswide referendum on divestment of the school's South African-related securities . . . BRITAIN'S QUEEN ELIZABETH II has given her assent to legislation to permit the prosecution of alleged Nazi war criminals, the government says . . . POPE JOHN PAUL II flew to Portugal Friday for a four-day visit to be climaxed by religious rites at the shrine of Our Lady of Fatima, who the pope believes saved his life when a Turkish terrorist shot him in St. Peter's square a decade ago.