In Brussels, a 17-year-old Soviet musician has won the Queen Elisabeth international violin contest and will receive $7,000 in cash and a lucrative contract for concerts throughout Europe. Vadim Repin edged out another 17-year-old musician, Akiko Suwanai of Japan, in the competition that lasted a month. They were the two youngest contestants. Musicians from the Soviet Union, Israel and West Germany took third through fifth places, followed by Catherine Cho of Ann Arbor, Mich. Twelve finalists performed works by Mozart, Beethoven, Brahms and Debussy with Belgium's National Orchestra to decide the $37,300 in prizes. The grueling competition began in early May with 53 violinists.