I would advice you contact the British Council in those Countries (Thailand or Vietnam) and they would be able to provide further information about teacher training etc.
Thailand Teacher Training: http://www.britishcouncil.org/thaila...evelopment.htm
Vietnam Teacher Training: http://www.britishcouncil.org/vietna...r-training.htm
If the British Council in those countries don't offer the CELTA Course, they probably know a reputable provider for the Course and offer advice about reputable employers in the country.
Hope that helps.