|A map of "Khmer Krom," territory once dominated by Khmer speakers before it was conquered by Vietnam in the 18th and 19th centuries.|
Image Source: Douc Sokha, "សហគមន៍ខ្មែរក្រោមថារកឃើញឯកសារជាង៤០០០ទំព័រ ទាក់ទងនឹងការកាត់ទឹកដីកម្ពុជាក្រោមឲ្យវៀតណាម", Vod Hot News (15 February 2015)
There is a great deal of truth to that narrative. However, I argue that is one an element critical to Sino-Cambodian ties that tends to get overlooked. It will remain a factor regardless of who is running the show in Phnom Penh: the Vietnamese.
Here are a few quotes:
Ethnic disharmony is not hard to spot in Southeast Asia, but few of its prejudices — outside of the Myanmese hatred toward the Rohingya, at least — can match the distrust and disgust the average Khmer feels toward the Vietnamese. Recall how conservative Americans talked about the Soviet Union at the height of communist power, add the way their counterparts in modern Europe discuss Arab immigration now, and then throw in a dash of the humiliation that marked Germany in interwar years, and then you might come close to getting a fair idea of how wild and vitriolic a force anti-Vietnamese rhetoric is in Cambodian politics....and then the conclusion:
Although both Vietnamese immigration and government influence has waned since Hanoi ordered its troops to withdraw from Cambodian territory, distrust of Vietnam’s government and disgust toward Cambodia’s Vietnamese minority remain. You can see this even in the Khmer communities of the United States. To walk the streets of an American Cambodiatown is to see a half-dozen posters warning of Vietnamese aggression, or (if you speak Khmer) be pressed to attend activist get-togethers or donate to help fight Vietnamese imperialism.
Many of these donations go straight into the coffers of the Cambodia National Rescue Party (CNRP), the opposition to Hun Sen’s ruling regime. The CNRP faces a stacked deck when squaring off against hostile authorities, but anti-Vietnamese agitation is a game they can’t lose. When the Vietnamese overthrew the Khmer Rouge, the man they chose to head their new puppet regime was none other than Hun Sen. The party he now heads is a direct descendant of the party the Vietnamese created to rule Cambodia. While Westerners sometimes call Hun Sen a Chinese puppet, his domestic enemies are far more likely to attack him as a Vietnamese figurehead....
The United States, a longtime ally of the Thais and newfound courter of Vietnamese affection, could not be trusted to put Cambodian interests above the other powers in the region. In Beijing, the Cambodians see a more reliable great power — an ally that not only has a fractious relationship with Cambodia’s traditional enemy, but one that has demonstrated a willingness to go to war with that country to preserve a favorable balance of power in Southeast Asia. Indeed, the last war China waged was not only against the Vietnamese, it was against them in defense of Cambodia. Beijing’s decision to send troops across Vietnam’s northern border as the bulk of the Vietnamese army was fighting an insurgency in Cambodia, and then to keep a threatening military presence on that border through the next decade, badly hampered the Vietnamese push to become the premier armed power in Southeast Asia. For Cambodia, the strategic benefits of friendship with China could not be clearer. Playing spoiler in ASEAN meetings is a small price to pay to guarantee this friendship. 
In Cambodian terms, Hun Sen’s decision to tilt Cambodian foreign policy toward Beijing is quite moderate. Other voices in Cambodian politics advocate even closer ties to China in hopes of generating more leverage vis-à-vis the Vietnamese. Rainsy declared in 2014 to a group of CNRP party supporters that his party is “on the side of China, and we support China in fighting against Vietnam over the South China Sea issue. … The islands belong to China, but the Viets are trying to occupy them, because the Viets are very bad.” He would later defend these comments in a post on his Facebook page, arguing, “when it comes to ensuring the survival of Cambodia as an independent nation, there is a saying as old as the world: the enemy of my enemy is my friend.”
The CNRP, acutely aware of its image in Western circles, has since distanced itself from Rainsy’s comments, but his logic is solid. If Vietnam truly does threaten the sovereignty of Cambodia, closer relations with China is a geopolitical imperative. Cambodia’s politicians have depended, since French colonialism if not earlier, on foreign sponsors. But being tarred as a friend of the Vietnamese is the most toxic slur in Cambodian politics. For Hun Sen or Rainsy, leaning toward China doesn’t send a message of dependence on Beijing, but of hostility toward Hanoi.
Even radical changes in Cambodia’s internal politics are unlikely to produce a revolution in Cambodia’s foreign relations. Hun Sen’s patronage machine requires huge influxes of money to maintain. China provides that. It does so without asking Hun Sen to protect the liberties of average Cambodians in return. But even if the machine were to fall apart and the opposition were to rise to power, Cambodia’s new leaders would face strong political pressure to give Beijing pride of place.
Cambodia is a small country tucked between its historical enemies. The grip anti-Vietnamese sentiment has on the Cambodian masses only strengthens this geopolitical anxiety. As long as Cambodian nationalism defines itself in opposition to the Vietnamese, Cambodian politicians will never stop searching for a great power that can stand as a bulwark against Vietnam. For the foreseeable future, that country will be China. Next to this, the perceived balance of power between China and the United States will never be anything more than a sideshow. (emphasis added, hyperlinks not included) 
I encourage you to go over to Foreign Policy and read the whole thing.
One of the themes that I touch on in this piece, but don't fully develop for reasons of space, is that we sometimes focus too much on the grand drama of great power rivalry when looking at regions like Southeast Asia and don't narrow in on the smaller domestic pressures that might force politicians to choose one great power over another. This is probably because most analysts who focus on things like Sino-American rivalry don't have much experience or interest the domestic political squabbles of small countries on the Pacific periphery. But this is and always has been a major part of the 'why' behind who joins one side or another in great power competitions. It is a pattern that stretches all the way back to the Peloponnesian War. If you think the Bangkok's decision to work more closely with the PLA, or Duterte's unremitting efforts to undermine the U.S.-Philippines alliance have nothing to do with the domestic political economy of each country, then you are foolish. There is much more afoot here than a simple calculation of Chinese and American power, and if we refuse to recognize this we will be continually blind-sided by events to come.
Some folks have suggested on twitter that it is a bit silly to call Hun Sen "hostile" towards Hanoi, and I agree with this. Hun Sen is not hostile towards the Vietnamese--but he does benefit from appearing to be so when the occasion demands it. This is Hun Sen's special skill: an ability to appear to be exactly the person his wants his audience wants him to be. I cannot think of very many other actors on the international stage who are as talented at, well, acting, as he is. The best way to judge Hun Sen, then, is from his record. That record suggests Hun Sen has long been accustomed to accommodating Hanoi, while slowly building up the strength of Sino-Cambodian ties in the background in case Hanoi ever asks too much of him.
The CNRP is harder to judge, for the simple reason they have never actually been in charge of Cambodian foreign policy, and thus have no real track record to judge from. All we have to go on for them is rhetoric, and as the article notes, that rhetoric is mixed. I reached out to the CNRP to get a clarification of what official party policy is at the moment. Monovithya Kem, Deputy Director-General of Public Affairs for the CNRP, sent me a response, but it came too late to make it into the article's final draft. As I had extreme trouble finding any statement from the party on the South China Sea problem issued in the last year, it will be a public service to publish :
The CNRP official position on foreign affairs is a non-alignment one, meaning Cambodia will not be a client state to any foreign power. Our position on any international matters would take into account regional security and Cambodia's interests. We believe in the empowerment of ASEAN to be a stronger institution so that through this platform ASEAN nations can address critical regional issues together. 
 Personal correspondence with Monovithya Kem, dated 3 January 2017.