Which means, Cambodia never became a Soviet asset or was in any way beneficial to the Soviet Union (and this was the main goal of the Vietnam war: to prevent Soviet assets from appearing in the Southeast Asia). Pot Pot's regime was even a detriment to the Soviets as it tainted the 'good name' of Communism in the eyes of Western intellectuals.

It didn't turn Communist. Pol Pot's regime wasn't "communist" except calling itself this way. Soviet Union never recognized them as Communist.