Will I be able to use these in Vietnam and Cambodia, when on holiday there this November?

Probably not. Much better to take an ATM card with a few dollars cash as back up. Very few places will accept TCs these days. Cambodia ATMs dispense USD as it is the de facto currency there.

If you are visiting Siem Reap and PP in Cambodia, note that the US Dollar is the currency of choice. Bring plenty of small bills.

In VN, the Dong is the currency of choice.

Finding and using ATMs in both countries is very easy.

Cashing US dollar denominated travelers checks will not be easy and if you can find a place that will cash them, expect to pay for the privilege of cashing them.

Travelers cheques are difficult <i>anywhere</i>.

Very 20th Century

In fact few banks even sell them any longer.

The money changers will cash them in Siem Reap, but charge a stiff premium. You are better off, as said above, using an ATM card or a credit card that has no (or low) foreign exchange fee.
In Vietnam, remember that the Dong are pretty much worthless once you leave the country, so try to estimate when withdrawing. I found it helpful to figure out how much I needed before I started the process at an ATM. Somehow three million Dong sounded like I was making a mistake unless I had done the calculation beforehand (It's only $200 USD) Vietnam.

Cambodian ATM's usually dispense US Dollars.

The only problem with Cambodian ATMs is that they usually dispense $100 bills. As Kathie says, take plenty of small bills. A lot of places do not want to take large bills although I've had women in the old market leave me minding the shop and take off somewhere with a $100 bill and come back with the correct change. Money changers will change them but charge a dollar or so for the privilege. You've got to laugh at some of the money changers, we were going up to Phnom Kulen last year and my daughter got $15 changed into 100 riel bills for the beggars. They tried to charge her extra because they were new notes!