Lafoto wins again!!!!
Yep, they're from Cambodia. I went there in 1996 during some general travels around asia. The sculls are quite common. They're remains from the late 70s, early 80's Khmer Rouge Pol Pot regime. They established "year zero" and wanted to start a socialist society from the beginning. Anyone with education, etc, was said to be killed. There is a lot of evidence still.
The ruins are from an earlier Khmer civilization at Angkor (similar Khmer ruins can indeed be found in Vietnam). Built around 1300 AD it was a massive city of over a million (the second such after Rome). It was Buddhist (as Cambodia is today) and only the temples and royal buildings were of stone and survive today. The civilisation completely collapsed and was lost to the rapacious jungle, only to be found in the 19th century by a French explorer.
I was there for a week just after the civil war and had the place completely to myself. It's really awesome.
So there you go.