So a provocative claim with no proof?The US was reputed to have set up a complete "shadow Government" in it's embassy in Saigon. It handled all major decisions made to run South Vietnam.
Lets look at the facts. The United States provided material aid to South Vietnam, not unlike the Soviet Union and China. It also provided training support, something the other communist states also did.
The United States helped pay for policies and programs the government of the republic of Vietnam did. This makes sense since how can a broke nation pay for programs intended to modernize and develop its nation? This is no different to what the United States did in Taiwan, Japan, and South Korea. Unless you believe they're run by secret CIA in embassies too.
Famously, American military advisers and policymakers were furious and angry that their Vietnamese counterparts frequently didn't listen to them. Doesn't this throw a wrench in your secret puppet government claim? Eisenhower and Kennedy were furious that Diem never listened to them and that was one of their main complaints. Frequently throughout the 1960s, the United States advocated appeasement with the various fighting factions inside South Vietnam. Yet, the period with the most political, military, and economic stability in South Vietnam was when leaders didn't listen to this American advice. Those were the two longest serving presidents in South Vietnam's history.