An interesting article from VOA News:

Anti-Vietnam Fears Stoked Ahead of Cambodia Election

The article notes that the last time this strategy was used was in the 2013 election:

It paid dividends in 2013, when the Cambodian National Rescue Party (CNRP) performed well at general elections, sharply reducing the number of seats held by the long-ruling Cambodian Peoples Party (CPP) in the National Assembly. Neither party responded to VOA’s request for comment for this story.
“The strategy that politicians use to gain popularity, by condemning Vietnam over invasions here and using insulting words is just not right,” said Muoy Piseth, a spokesman for the Federation of Cambodian Intellectuals and Students.
He added that as a result, ridiculous levels of hate are being passed from one generation to the next.

It is also, as the article notes, not just something that a matter of foreign policy, but something that is directed against the Vietnamese immigrants living in Cambodia:

Real or not, those fears are a factor in Chbar Ampov, a district in Phnom Penh with the highest concentration of ethnic Vietnamese in the capital.
Here it is difficult to distinguish between Khmers and Vietnamese because most people speak Khmer fluently, dress Khmer and have adopted Khmer culture.
Despite their assimilation, Chea Ny, a middle aged street drink seller, said she generally dislikes Vietnam and claims most who live here are illegal immigrants who sneak across the border on boats and venture up the Mekong River to Phnom Penh looking for work.
“I hate them. I hate them for messing around in our land. They steal our jobs and the government is doing nothing about it,” she said, at times using a derogatory Khmer word used to describe Vietnamese. 

And it is something that affects the young Cambodians in particular:

Analysts said anti-Vietnamese sentiment also plays well with the youth vote, who are expected to figure prominently at the polls. Two decades of peace has spawned a baby boom, with 70 percent of the population under the age of 30 and they backed the CNRP four years ago. 

I've been outside of Cambodia for two years now, but I was living in Cambodia during the 2013 elections, and I remember the anti-Vietnamese sentiment that was prevalent at the time.  Students would even make anti-Vietnamese remarks in my classroom.
At the time, I didn't know how to respond to it.  In part because I was under the influence of Pedagogy of the Oppressed by Paulo Freire (W), a book which--if I'm being perfectly honest--I never actually read, but was familiar with by reputation and discussion.  Based on my understanding of the book's theories, I did not view it as my place as a teacher to tell the students what to think.  My job was only to find out what they were concerned about, and help them to better articulate their own concerns.
Therefore I didn't know what to do when the students started articulating such racist views.

In retrospect, I've regretted ever since that I didn't take a stronger stand on this.  I've often in my head mentally rehearsed the speeches that I should have given to my Cambodian students about how not to listen to politicians when they tell you that other groups of people are your enemy.