I've been reading McMaster's Dereliction of Duty on the USA's invasion. My impression is that McNamara's primary goal was to demonstrate to the rest of the world that the USA was committed to anti-Communism, so as to prevent future communist insurrections or proxy wars. Sustaining casualties in Vietnam was one way to do that. Preserving South Vietnam would have been a nice bonus but was not expected.
So, did the USA accomplish its war aims in Vietnam? Did they make themselves look fanatical enough to affect communists' decision-making?
No, the United States did not accomplish its initial (1965) aims, although historians argue that these sometimes abstract aims did change. One such argument, for example, turned final period of the United States invention into a war to secure and bring home American PoW. But for this question, I will focus only on the initial aims.
Now, you're not wrong in your assessment of McNamara's aim (although it was only one of three large aims that the United States had in South Vietnam). Where you do go wrong is the concept that it was meant to be a deterrent gesture. The United States was meant to show to the world (or rather, to their allies) that they were a reliable ally. That in times of threat, they would be there to help them fight communism. If the US had left South Vietnam to its own devices and allowing it to be run over by North Vietnam, what kind of ally would they have be? To put it simply: the credibility of the United States was at stake.
The two other reasons were that the United States was there to protect and uphold South Vietnam's independence as well a more general aim to combat communism as well as to prevent communism from spreading to the rest of South-East Asia.
This leaves us with your question: Did the United States accomplish these aims?
It's hard to tell because semantics makes it possible to argue that in one way, the United States did accomplish to protect and uphold South Vietnam's independence throughout the period of escalation of war (1965-1973), but as we all know, it didn't take long after the Americans left that the South fell to the North. The United States certainly showed itself willing to protect and fight against communism, but their credibility took a hard hit after the United States held back from intervening again in 1975 to prevent the fall of South Vietnam. The Vietnam War left a very messy public and military legacy which discouraged interventions abroad on the scale of the Vietnam War.
On the question of communism spreading to South-East Asia, it didn't really come to fruition despite it being the belief at the time that it would. It's clear that communism did spread to Laos and Cambodia (Kampuchea), but it didn't reach further than that due to other local conditions that weren't tied to any American initiatives in South Vietnam.