What was the relationship between the Vietnamese and Cambodian people during the Vietnam war? Why were the Vietnamese invading Cambodia?

Hello, I think I can give a pretty decent answer for you. You are asking about Viet Nam War (Second Indochina)-era relations, but I think I should give a bit of background on the relationship between the Khmer (Cambodians) and Kinh (Vietnamese) since old history played a role in the interactions during the Indochina Wars. For this background, I will be primarily using Nguyen Cochinchina: Southern Vietnam in the Seventeenth and Eighteenth Centuries by Li Tana. If you want to dive more into Viet Nam's expansionism or southern culture in general, I highly recommend this book, or at least a general Vietnamese history, such as the recent one by Christopher Goscha.

Actually, Goscha has said that the modern Viet Nam is a product of Vietnamese colonial dominance, just as they were colonized by the French. This is because during the time period that Li's book covers, Viet Nam defeated, for good, the competing empires of Champa and Angkor, securing the Mekong Delta for themselves. It didn't stop there. Up until the point that the French invaded Viet Nam in 1858, Viet Nam had been gaining more and more territory in what is now modern day Cambodia and Laos. In addition, Siam was conquering Angkor's territory from the west. Angkor was slowly being squeezed out of existence, and in fact the French, by coincidence, "saved" (in terms of politics; obviously colonialism was and continues to be devastatiing to Cambodia) Cambodia by invading the region and putting a halt to Vietnamese expansion, which had actually gone north into Qing territory as well. In any case, since this conflict, Cambodia has maintained a rather aggressive stance regarding Viet Nam and Vietnamese people, one that still exists to this day to a fairly fanatical degree, frankly.

That being said, during the time of revolution and the Indochina Wars, Cambodia was on decent terms with Viet Nam. Prince Sihanouk was in fact educated in Sai Gon and his two most noteworthy friends during that early period were Vietnamese and Chinese. For nearly 30 years, Sihanouk was the dominant political figure in Cambodian politics, and along with it came a relative accepting view of the Vietnamese communists, who were of course in the same party as the Cambodian ones. There were a few reasons for this, a major one being his hatred for colonialism (despite him being a Francophile) and in particular his hatred of Bao Dai, the final emperor of Viet Nam and head of state of the Associated State of Viet Nam, which had been organized by the French to challenge the communist Democratic Republic of Viet Nam. Sihanouk saw the government based in Sai Gon as colonialist in nature, and Bao Dai as a puppet. He also was a bit more sympathetic to the communists because of their oppostion to the West. In any case, he was also determined to do two things, both of which led to his eventual downfall: 1. Keep his hold on Cambodian politics and the imperial order and 2. Keep Cambodia neutral at all costs. These were not very popular positions among either hardline leftists (such as Saloth Sar, aka Pol Pot) or rightists (such as Son Ngoc Thanh), though as I said above he was definitely on better terms with leftists. His positions were also unpopular witht he US, both because of his leftist views but also because of his antagonistic relationship with the US, which included allowing DRVN soldiers and NLF fighters to cross into Cambodian territory on their way to the south. He did not like the US and felt they were trying to colonize the region, a view actually shared by prominent Vietnamese leaders, including Madame Nhu in Sai Gon. Anyways, basically Cambodia and Viet Nam had a strained but decent relationship during the revolutionary period and first two Indochina Wars because their goals were aligned.

The Vietnamese invaded Cambodia much later, in 1978. This was due to the Khmer Rouge deciding to attack Vietnamese villages, essentially invading deep into Vietnamese territory, as well as targeting ethnic Vietnamese during the genocide in Cambodia.Viet Nam decided to counterattack and remove the Khmer Rouge from power. Whether or not this action was humanitarian in nature is debatable, but in any case, Viet Nam held effective control of Cambodia until 1991. Still, such was the hatred of Vietnamese people that many, including Sihanouk himself, argued that the Khmer Rouge was still the legitimate government of Cambodia, despite the genocide. This is still the case in modern Cambodia, the denouncement of anything Vietnamese (including linguistic and cultural/ethnic/etc ties), even if the Vietnamese, whether intentionally or not, "saved" Cambodia from self-destruction.

Sources:

Sihanouk: Prince of Light, Prince of Darkness by Milton Osborne

Brother Number One: A Political Biography of Pol Pot by David Chandler

Nguyen Cochinchina: Southern Vietnam in the Seventeenth and Eighteenth Centuries by Li Tana

Vietnam, A New History by Christopher Goscha