From what I understand, they collapsed due to economic reasons. According to Wikipedia, the US withdrew around millions of dollars in support, but did that mean that the South Viet gov was ENTIRELY dependent on the aid of another country?
Note: I also read that a spike in oil prices further hastened their decline.
South Vietnam was entirely dependent on the US to answer your question. The vast, vast, vast majority of the South Vietnamese supported the communist North. In 1954, President Eisenhower wrote:
I have never talked or corresponded with a person knowledgeable in Indochinese affairs who did not agree that had elections been held as of the time of the fighting, possibly eighty percent of the population would have voted for the Communist Ho Chi Minh as their leader rather than Chief of State Bảo Đại. Indeed, the lack of leadership and drive on the part of Bảo Đại was a factor in the feeling prevalent among Vietnamese that they had nothing to fight for.
The historian James Gibson has a good quote on the situation during US involvement in the war:
Strategic hamlets had failed…. The South Vietnamese regime was incapable of winning the peasantry because of its class base among landlords. Indeed, there was no longer a ‘regime’ in the sense of a relatively stable political alliance and functioning bureaucracy. Instead, civil government and military operations had virtually ceased. The National Liberation Front had made great progress and was close to declaring provisional revolutionary governments in large areas (James Gibson, The Perfect War: Technowar in Vietnam, p. 88).
It was a corrupt oligarchy, and an unsteady one. Warlords in Africa can rely on raw minerals/blood diamonds and despots in the Middle-east can rely on oil to buy the loyalty on the army, but that was never an option for South Vietnam. It turned to US foreign aid, collecting 2.3 USD billion alone in 1971 (Wiest, Andrew A., The Vietnam War, 1956–1975, p. 80).
Then the aid cuts off. Huge amounts of US soldiers left the country. Industrial output shrank by 20% in 1973 and 1974 (Đặng Phong (2004), Kinh tế Miền Nam Việt nam thời kỳ 1955 - 1975, Nhà xuất bản Khoa học xã hội, Hà Nội, tháng 12). Then we hit the spike in oil prices you mentioned.
Maintaining the military became impossible. Maintaining the government became impossible. You can imagine how quickly South Vietnam fell after that.