The Vietnamese communists in the early seventies will want a protective girdle of satellite states around them so i can't see them tolerating an unreliable non-communist monarch like Sihanouk for long.
Didn't Vietnam invade Cambodia to depose Pol Pot?
They only did that because he tried to reclaim historically Khmer territory around the mouth of the Mekong River in what is now southern Vietnam. Had he not, his regime would still be ruling Cambodia (or, as they called it, Democratic Kampuchea) to this day. And in a way, it still is, considering his former followers still rule the country.