I'm reading that the Cambodian Riel is not as widely used as I would expect, and has only caught on as a local currency in markets after the Khmer Rouge dissolved in 1980 (according to Wikipedia). My tour enters Cambodia from Vietnam at Phnom Penh then I only visit Angkor Wat before hitting Thailand, so I'm only there for 6 days or so.
Can I exchange my remaining Vietnam Dong to Cambodia Riel at the border, can I get Riel out at an ATM in Phnom Penh and carry a few US dollars on me, or should I just use US dollars for my whole stay? I want to be respectful to each country's currency and culture, but if it won't be of any use then I won't bother getting any. I'll only really be buying food, drinks, and market souvenirs.