I live in Thailand and have traveled to all the countries you will visit. Traveler’s checks are not the way to go here at all. I suggest an ATM card, as it is safer and you get a better exchange rate. While you might use a credit card in some hotels, this is an area of the world where cash is king. You can exchange Euros, Pounds or USD at the money changer’s, but not a good rate. Stay away from the banks as well. ATM has been the best way, but have back-up. Oh don’t have your hotel exchange cash either. 40 pounds should be good, as you will find food really cheap for local food on the street or in small shops. A guesthouse or hostel are cheap. In Vietnam, Thailand and Cambodia guesthouses are pretty nice. Use to live in Ho Chi Minh City and there are some great family owned guesthouses. Transport is really cheap in country. You will need some money for Visa’s at the boarder. It’s really the bargain center of the world and you will have a great time!!!!

Right there are quite a few ways. Traveller’s cheques are generally going the way of the dodo to be honest. They just aren’t worth it.

The best thing to do is have multiple options. Have say a weeks worth (for example) of local currency on you when you arrive. Have small notes too, with maybe £20 in GBP as back up.
Prepaid cards are a good option (some of these have fees associated too so shop around) pre loaded with some money that you can use in some (not all, depending on the card) ATMs. Plus your normal debit card for use in ATM’s too. Then have a credit card (stashed safely and seperately from your other cards and money for emergency use.

Personally I have always just had a bit of cash, used my debit card periodically and had my credit card as back up.

Get the Travelex Cash Passport Globe travel card before you go. Load it up beforehand – it’s in British pounds (they don’t do travel cards in Thai Baht) but this card has the cheapest charges overall for using abroad. Your current ATM cards will charge a fee (up to 4%) plus give an amazingly bad exchange rate. Passport Globe charges 99p per ATM withdrawal, way cheaper than the majority of credit cards issued by banks. Get one (and more details) from the Travelex website – I’ve used them many times. Other than that, carry some back up Euro or US dollars, both are easily changed in SE Asia.

US dollars are popular in both Viet Nam and Cambodia and can be used throughout. VOA for Cambodia and pre-approved Visa for Viet Nam must be paid in US dollars. Remember, both the dong and riel are inconvertible currencies so try to use them before departing.

I usually take about $2,000 in cash – some in US and AUD and change some on arrival. When using ATM’s overseas, I withdraw enough to last me for about a week, because of fees. There is a 150 baht fee for ATM transactions in Thailand except for AEON ATMs. I also take a credit card and Visa card and keep money/cards in separate pockets. Can’t help with India as I’ve not been there.