SASLess, most of Vietnam lies east of 105 deg East. Under the SEATO the British sphere of influence extended west of 105 deg East and the US east of that meridian.
Obviously were the minor parts of a country crossed that meridian it would be in the sphere of influence of the major part, ie the whole of Vietnam would fall within your sphere.
In late 1962 Indonesia laid claim to parts of Borneo in an attempt to break up the new state of Malaysia. Britain and ANZ forces reinforced Malaysia. In late 1964, when the level of confrontation increased a significant number of additional British and Commonwealth units were deployed there.
In the case of the UK many of these forces were already NATO assigned so there was considerable military stretch and the new Government was intent on reducing our commitments and the concomitant costs.
On 7th December 1964, Patrick Gordon Walker, Foreign Secretary and Dennis Healey the Secretary of State for Defence briefed the US Secretary of State Dean Rusk and Secretary for Defence Robert McNamara on their intention to run down the size of the UK’s armed forces over 10 years as defence spending would be maintained at its present levels. The Secretary of State urged Britain to maintain its position on the world stage as we could do things that the US could not, or did not, want to do.
That we stayed out of the Vietnam war was with the tacit acceptance of your Government as we were already committed to maintain stability in our sphere of influence.