As the previous post suggested, the terms "right" or "wrong" might not be entirely applicable here. Instead, the discussion might be more along the lines of whether or not the perception was understood. I think that the perception was that the Americans were another imperialistic force that sought to control the Vietnam nation as the French did before and as others had prior to the French. The same way the Viet Cong held perceptions about American attempts at imperialism, the Americans perceived the Viet Cong as being a Communist tool of the Chinese or the Russians. That might not have been right, as well, but it was perception of the time. In the same manner that the Viet Cong perceived the United States threat of colonial power, the American threat of a Communist domino theory was asserted. Both might have been the misread of political governments at the time.
I don't know if they were right or not, but you can certainly see why they might feel that way. While the US was not there as a colonial power like the French, they were there to contain the spread of communism, and while doing so, spread the use of English, democracy and capitalism, or at least, that was the plan.
As far as how the VC saw us as an enemy, it also makes sense that they would view us as successors to the French. The Vietnamese had running battles with China for centuries in early history, then the French in the mid-19th century, the Japanese from 1941 - 45, and then the French again from 1945 - 54. We were just the latest in a long string of colonizers, or at least, that's how most Vietcong and others saw the situation. Theirs was a war of independence, and we were just the latest empire to stand in their way.
From the standpoint of Vietnamese history, I'm afraid they were right. Not that we intended to remain there permanently, or to run their government and country like the Chinese and French had. But we were in the way of their civil war, as the French had been before us. The war between South and North Vietnam was simply a continuation of a war which had been going on for two centuries.
The state of Dai Viet was set up by a renegade Chinese general in 213 BC, establishing his base in what became Hanoi. Just after 200 BC the Chinese invaded, beginning an occupation which lasted until the mid-10th century. In that thousand-plus year war neither the lowland Vietnamese (of Chinese origin) nor the other ethnic groups ever accepted China's rule. The war continued from generation to generation until they drove out the Chinese.
The northern (ethnic Chinese) Vietnamese people kept up their own war of conquest against the mountain peoples and the kingdoms to the South, expanding their control into the Central Highlands and to the southern tip of what became known as Indochina. They also fought wars with the Khmer, Thai and Lao kingdoms. Eventually Vietnam was in area much the same as today, and a unified country. Unfortunately, soon (1776) a civil war began between the Trinh family in the north and the Nguyens in the south. The Nguyen established their dynasty and built the imperial capital of Hue, but the civil war never really ended until 1975.
The French arrived and took over, but never established the degree of control they had in other colonies. The civil war between the north and south bubbled along under the surface. The Japanese came, and then the US Army sent a military training mission from the Office of Strategic Services in 1945. They trained and led the Viet Minh, a new army raised by a history professor named Giap. When the war ended later in 1945, Lord Mountbatten (Allied commander in the CBI theatre) decided to use the Japanese troops as police instead of disarming them, and to back the French in their re-seizure of the country. The OSS team at first led the Viet Minh against the French, but were withdrawn and the US allowed our allies to reestablish their colonial control.
Giap, of course, became the head of the People's Army of Viet Nam (PAVN) under the political leadership of Ho Chi Minh, who asked the American government for recognition and support of a national government. The emperor, Bao Dai, abdicated in favor of Ho, but the French brought him back as a puppet. The fact that we allowed this to happen was ultimately disastrous. Ho's real name was Nguyen That Thanh, and he was a relative of Bao Dai, who was a Nguyen.