Will no-one post/courier the supplies to Vietnam? That would surely be cheaper than a flight.
If I'm looking at the right advice [1], insulin is £1.50-£3 ($2-4) in the UK, so that may be one possible source.
(The UK suppliers I found don't have information on overseas delivery on their sites; it's probably worth a phone call.)
[1] http://www.diabetes.co.uk/insulin-pumps/buying-an-insulin-pu...