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Oxford Seminars is recognized amongst the TEFL/TESL world and has a reputable reputation.
The CELTA is the certificate most English language teaching institutions prefer in their job advertisements if they do not qualify their ads with "OR" other equivalent certificates in TEFL.
If you are also considering doing the CELTA and doing it while in Vietnam that option is probably your best because Vietnamese English language teaching institutions will instantly recognize the certificate and know its value, and you can probably be employed quickly after graduation which is what you want - teaching abroad.