The US was bombing South Vietnam very intensively from the early 60s, and many people, and South Vietnamese resistance forces, fled to border areas, sometimes spilling over to border areas of Cambodia.
So in that sense there were “VC installations” there. If one takes for granted that the US has the right to attack and destroy South Vietnam, then “VC installations” in border areas of Cambodia are criminal aggression. The US is making the same charges now against Iran: it is accused of supporting the resistance to the US occupation of Iraq (on its border, a state that attacked Iran with US support under Saddam Hussein, causing huge casualties and damage). The accusation makes sense, on the assumption that the US has a right to invade and occupy Iraq and no one has the right to interfere. Hizbollah is also condemned for terrorism because of its actions to drive Israel out of southern Lebanon, which it had invaded and was holding in violation of Security Council orders. Other violent states make similar arguments.
The US supported an invasion/coup to overthrow the neutralist government in Cambodia in 1958. Pretty much the same forces carried out the US-backed Lon Nol coup in 1970. George Kahin, the founder of US Southeast Asian studies, wrote about this in his book Subversion as Foreign Policy (with Audrey Kahin), mainly about Indonesia The US was continually bombing border areas through the 60s. In 1969 the bombing became far more serious. Cambodia issued a White Paper giving details of US bombing attacks, and Prince Sihanouk pleaded with the international press, in a press conference in Phnom Penh, to publicize and protest the bombing of neutral Cambodia, which was aimed at defenseless Khmer farmers and villagers, not at the Vietnamese clustering in border areas, after being driven out of their country by the US invasion. All of this was totally ignored. I wrote about it at the time, as did some others well outside the mainstream. By now there is a fair amount of literature.