Exxon Corp. has dashed Vietnam's hopes of developing offshore oil resources by withdrawing from exploration talks and shutting down its offices in Hanoi. The move was prompted by a Vietnamese refusal to accept product sharing, insisting instead on creating a joint venture in which it would have a 50% share. Irritation with Vietnam's approach has already decreased the number of foreign oil license holders from 20 to 12 since 1992. Mobil Corp., which is expected to merge with Exxon soon, is likely to join the exit queue as well.