There is an office of Vietnam embassy in Bandar Seri Begawan, Brunei.
Up to now, Vietnam’s embassy in Bandar Seri Begawan is the only Vietnamese representation in Brunei. This office provides services of visa issuance and consular legalization among others, and also the place that Vietnamese citizens in Brunei should contact immediately if they lose their passport.
Regarding visa for Vietnam, in addition to embassy visa, citizens and residents of Brunei has an alternative of getting a visa on arrival. You can see their brief below:
For details, you may also read: