For the reasons Pontius Navigator pointed out in post Number 3 there was no RAF involvement in Vietnam combat after 1964. There was however RAF involvement prior to December 1964.
The existence of the South Vietnam clasp to the 1962 GSM is also a bright red and very whiffy herring.
Archimedes is correct in post 17 when he points out that all the clasps were awarded to early members (or defacto members) of the Australian Army Training Team Vietnam. Some of whom originated from the UK and some of whom had previous service with British Armed Forces.
From an aviation perspective a few of the SVN Clap recipientsare significant in a couple of pioneering respects with regard to helicopter operations. Captain Noel Delahunty was awarded the Military Cross for conducting the first 'Commonwealth' hot extraction Warrant Officer George Chinn was awarded the Distinguished Conduct Medal in the same year for taking charge of the first hot insertion.
As Archimedes has previously pointed out some of the 'Australians' were British. Warrant Officer Brian Quee born in London in 1930 after serving a tour as an instructor with AATTV from 1962 to 1963 he returned on his second tour as
Squadron Sergeant Major of 161 Reece Flight. 161 Recce Flight operated Cessna 180's and Sioux Helicopters in support of the 1st Australian Task Force at Nui Dat from 1967 to 1968.
All the 68 awardees of the SVN clasp
are known and documented. The number of awards is sometimes misrepresented as 60 or sometimes 70 (as has already occurred on this thread) This is more readily attributed to a rounding error rather than any 'secret' RAF service in Vietnam.
There are some sound reasons why some RAF , RN and Army working in the embassy in the period 1965 to 1966 may have erroneously believed that they had been awarded the SVN clasp.
To answer the question proposed by the OP - Yes. Prior to 1964 at least one RAF helicopter was killed (and still missing in action) while seconded to the DARPA helicopter gunship trials.
Regards
Mick