TEFL International's PELT Cert. TESOL might well be a solution

The course is divided into 3 sections:

1. 4O hours of tuition with a trainer

2. a 23 DVD HomeStudy Pack with input from some of the world's top teacher trainers

3. a paid internship

Part 1 you wouldnt have to do as you already have substantial teaching experience

Part 2 covers as much if not more than an online course plus is a resource you can keep with you for as long as you are teaching

Part 3 can be done whilst you are working in Vietnam - they have a centre in Ho Chi Mhin city

The whole course meets international standards for a full certificate course and will be more widely accepted/respected than an online course