'South Vietnam' is an incredibly rare clasp, given to the small number of Australians serving there between 1962 and 1964. Later the Vietnam Medal was struck and given exclusively to Australian and New Zealand Forces
I seem to recall that our Antipodean friends were not highly delighted at receiving what was seen as an essentially British award, and particularly inappropriate in a campaign in which the UK was not officially involved. Hence the introduction of their own awards, and the use of the somewhat derogatory nickname given to the British GSM, namely the "Violet Crumble Bar", derived from a very popular Australian chocolate bar with a mainly purple foil wrapping.
Jack