Unfortunately I'm not well versed enough in the specifics of the war if you're looking for immediate causes, but I've seen the war's causes explained reasonably well on broader terms in the realm of Sino-Soviet politics. The situation was significantly more complicated than you seem to think it is. Common core ideology didn't preclude splits and hostility among Communist nations (as we see with Yugoslavia, Albania, and China), and, similar to the superficially utopian ideals of the French Revolution, Communist states tended to be pretty quickly confronted with the geopolitical realities that tended to conflict with their ultimate visions.

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In the context of Vietnam and Cambodia, this involves the Sino-Soviet split and, more locally, Vietnamese ambitions towards being the hegemonic Communist power in Indochina. Stepping back a few years, China began to grow distant from the Soviets in the years following Stalin's death, ultimately becoming a major rival to the Soviets in what became known as the Sino-Soviet split. Through the rest of the Cold War, that led to an interesting situation where the Chinese were often working closely with other non-Soviet-aligned Communists and working to undermine Soviet-backed nations where they could. During US involvement in Vietnam, the Chinese tended to play a delicate role in which they tried to undermine Soviet aid efforts for the North Vietnamese while not going so far as to let the North Vietnamese government collapse and produce an anticommunist regime on their southern border. When the war in Vietnam finally ended, China had failed to push the North Vietnamese away from the Soviets and into their sphere of influence, so, while they didn't have to deal with an American puppet on their border, Vietnam was still closely aligned with the Soviets, who they remained hostile. During this same period, however, the Chinese had been backing Maoist rebels in Cambodia in the form of the Khmer Rouge. The Khmer Rouge ultimately succeeded in overthrowing the government in 1975, installing their infamous regime.

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So by the latter half of the '70s, Vietnam had just finished reunifying and, given its legacy of extensive cross-border operations in Laos and Cambodia during the Vietnam War, generally had the sense of being the hegemonic Communist power in Indochina. However, Cambodia's extreme Maoist regime had designs on regional hegemony of its own, backed by the relative safety of having its patron power looking over Vietnam's shoulder while Vietnam's patron, the Soviets, was far too distant to provide any meaningful support in a timely manner. Things progressed into cross-border raids, and, as the Khmer Rouge regime descended into genocide, the ethnic Vietnamese in Cambodia became a target, as did villages across the border in Vietnam proper. China tried to mediate between the powers, but when the Soviets signed a treaty with Vietnam guaranteeing aid in the event of war with China, the Vietnamese finally moved in to oust the Khmer Rouge.

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While looking from a modern perspective, the Vietnamese seem to have a very noble casus belli (ongoing genocide, hostile violations of sovereignty), their actions weren't particularly well received by most of the world, and, given their installation of a pro-Vietnamese puppet regime, it's been argued by many that their intentions were more to consolidate power in the region than for any noble causes such as ending the Cambodian Genocide. With that in mind, it's no surprise that the invasion wasn't exactly well-received by much of the world. The US, UK and France were among those to condemn the invasion, and neighboring Thailand would openly provide refuge to Khmer Rouge leadership fleeing the invading Vietnamese. China was obviously angered, opening up a front on Vietnam's northern border. And while the Chinese performed less spectacularly than they may have expected in the north, ongoing clashes in the north kept a steady pressure on Vietnam's government and would prevent Vietnam from securing the kind of regional hegemony they seemed to be looking for.