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You cannot get it issued in Vietnam. It has to be done by post via .....https://www.gov.uk/government/world-location-news/changes-to-passport-services-in-vietnam
I got mine thru Hong Kong in 2014 I think - took about 4 weeks but that is no longer possible I gather.
6 weeks is a bit tight if you need to renew a visa every 3 months.