Australia joined the air war and sent,among other assets, a squadron of SAAB 35FA Drakens.
France and UK would be the most likely IMHO. Maybe Canada as well?
Many years ago, there was an article in Air-Britain's AeroMilitatria about RAF operations in Singapore using Gloster Javelins. A reader had sent in a letter to the editor, asking if there was any reports of Javelins making combat contacts with what was going on just to the north. Later, another person responded to that by saying he was a Javelin pilot at the time and that there had been an incident when one Javelin had returned with a missile missing from a pylon. The pilot said that everything was hushed up and no one got to know the for or why about the incident.
Now what if the UK had got involved in Vietnam --------
Now Mrs kitnut is an Army Brat, and her Dad (REME) was posted to Singapore around the same time as the beginning of the war. Mrs kitnut was 1 or 2 years old at the time and they were there for about 3 to 4 years.
If Canada joined now, I'd half expect them to be fighting for China
Maybe the Piper Enforcer version of the Mustang might have gotten built and issued to SV. Air Force."FarmGate II" to replace the original T-28 Trojan/AT-28 Nomad that were quickly using up the available flight hours on the aircraft planned/programmed service life.
Or perhaps France gets re-involved and thus one result might be Armée de l'Air and RAAF Mirage IIIs operating in Vietnam.
Most of the ideas shown up are either from the US and Allies sides - but how about the North Vietnamese? Shouldn't they receive some "love"
- Su-7 - taking a more direct ground attack role to the west
- MiG-23s - ditto above