Hi Rose,
I've done several holidays were I've had to catch a connecting flight to my final destination, and just to add to Mildred's reply, when you check in at Heathrow you will be given two boarding passes - one for your flight to Singapore and another for your onward flight to Siam Reap. On arrival in Singapore, you just follow signs for 'Connecting Flights' and make your way to your next departure gate at the appropriate time.
The same procedure happens on your return, so just make sure that you are given two boarding passes at Ho Chi Minh City.
Hope this helps and have a great time. I'd be really interested in your opinion of this holiday so maybe you could write a review on your return.
Mike