I think you bring up a very interesting point. I'll put a disclaimer that I am Vietnamese, so this may cloud my judgement.
I'm not sure if calling the government Vietnam put in place a colonial regime is accurate. No doubt, the actions of the newly installed Cambodian regime was placed under heavy scrutiny by the Vietnamese communist. However, I think this is no different than what the Soviets did to other nations in the Warsaw Pact. At best, you can say the Vietnamese attempted nation building similar to what the Americans did in Iraq. At worse, you can say it was a satellite state of Vietnam. I feel like calling it a colonial regime means that the Vietnamese were trying to exploit Cambodia for economical gains which isn't true. The war in Cambodia was costly and made the already dire economy in Vietnam worse. From my point of view, the Vietnamese wanted to put a friendly and stable regime to prevent the Khmer Rouge from returning to power and continue their attacks against the Vietnamese.
Now, I will try to explain this "colonial state" perception. To be honest, Vietnamese and Cambodian people have traditionally been enemies going back centuries. There is great distrust between our people. In fact, Southern Vietnam (Saigon and the Mekong Delta) originally belonged to the Khmer Empire and the Khmer people originally. Around the 11th century, the Vietnamese kings in the north decided to expand their lands through Nam Tien (Southern Advance). The Vietnamese moved south and conquered the Champa people and parts of the Khmer Empire. Along the process, we "Vietnamized" the region by bringing in Vietnamese settlers and often times massacring the local people. This is a dark event in history, but it happened in many other places around the world. In the 1800s, the Vietnam Empire under the Nguyen Dynasty invaded and conquered almost all of Cambodia. The Cambodian King sought help from Siam, but the Vietnamese also defeated the Siamese Empire. However, the Vietnamese decided to decide up Cambodia and give half of it to Siam to ensure future peace. The Cambodia/Khmer King decided to turn to France for assistance which was the first step of French colonization of all of Indochina. You can see why Cambodians distrust Vietnamese people, which leads to this colonial state idea.
There were many Vietnamese people who settled in Cambodia lands when the Vietnamese Empire existed. When the French took over, the migration of Vietnamese people to Cambodian lands continued. This led to a sizable Vietnamese population in Cambodia. The Vietnamese people who were already there in Cambodia (some are now many generation old) were treated as second-class citizens by newly independent Cambodia. Despite the ideological differences between the Khmer Republic (Lol Nol) and the Khmer Rouge (Pol Pot), they both believed in Khmer nationalism. Both were distrustful of the Vietnamese (the South Vietnamese for Lol Nol and the North Vietnamese for Pol Pot). Lol Nol actually massacred Vietnamese-Cambodian people during the Vietnam War and planned to invade South Vietnam to regain formerly Khmer lands. Pol Pot equally distrusted the communist Vietnamese and their cooperation soon ended before the fall of Saigon. Later, Pol Pot would also carry out violence against Vietnamese-Cambodians.
*Before posting this, I looked up the Wikipedia page on Vietnamese-Cambodians and found out there were some later Vietnamese migrants waves during the Vietnamese occupation. I guess this does give credit to the "colonial state" claim. However, I posted this anyways cause I've already talked about the history of Vietnamese and Cambodian people and I thought it would be cool for you guys to know.
