East of Suez was not chiefly about avoiding Vietnam

How much did avoiding Vietnam drive Britain's East of Suez decision?

Very little; the decision was primarily about Treasury pressure and reducing standing global commitments, even though Britain continued to deploy east of Suez afterward.

Britain was already avoiding direct participation in Vietnam while supporting anti-communist security efforts elsewhere in Southeast Asia. Keeping Vietnam separate from British operations in Malaya and neighbouring Commonwealth countries served both British and American interests. The East of Suez announcement answered Treasury pressure, while reducing the forces available to diplomats and weakening the standing associated with those commitments. Later governments continued to send forces east of Suez, financing operations through contingency funds or diverting defence money intended for other purposes. The policy therefore reduced the planned resources without ending the deployments that consumed them. It also signalled a transfer of the global-policing burden toward the United States.