The Khmer Rouge lose the war and never take control of Cambodia. How this happens doesn't matter (though you can add it if you want). Just what would Cambodia look like today? From what I saw, it was a modestly successful country and was on the rise before Pol Pot seized control. Would it be the South Korea of Southeast Asia or would it just be a modestly successful country?
I live in the region as a native and am somewhat more than familiar with the local history.
If there was no Khmer Rouge (maybe butterfly away?), then the replacement state would either be the Kingdom of Cambodia or the Khmer Republic (which was a defacto dictatorship). Regardless, the patterns of ruling won't be that different.
The biggest change would be the economy.
The Khmer Rouge wiped out 20% of the population but the 20% that was wiped out was the most educated and most skilled segment of the population. Teachers, academics, engineers, mechanics, technicians, doctors, professionals were all specifically targeted. People whom wore glasses were targeted and purged as wearing glasses was a sign of education.
Without the purge, Cambodia might possibly be a lot more economically developed; it won't be anyway near Thailand but still have significant manufacturing and industrial operations.
But I still feel that the Vietnamese would still have invaded anyway.