You don't pay departure tax in Thailand. It's now in the price of your tickets. Cambodia u pay in US$ I can't remember what we did in VN. If we did it would be in us$ and they are not that fussy about them being crisp dollars just make sure they are not completely knackered.

Smeagol's outta date. In Cambodia you don't pay anything now.

Yup, I know you used to - but it's changed. I just left there ten days ago. The counters are closed. No departure fee.

All of the various countries seem to be moving toward having the departure tax incorporated into the ticket price. Thailand did that a number of years ago, Burma did it last summer, and it sounds like Cambodia recently made the change as well. I don't know what VN is doing now, nor Korea. But everywhere in the world I've been where a departure tax was collected they required cash. Sometimes it's US dollars, sometimes the local currency, but I've never seen a place that allowed payment with a credit card.

Other countries do expect US dollars (for visa or departure taxes or for exchange to the local currency) to be clean and without tears. Only in Burma must the dollars be absolutely crisp.

We didn't pay any departure tax leaving Saigon airport last fall; it must be included in the ticket.