Since GAP is a Canadian tour company I assume that you are coming from there. I note that it is budget travel with hotels in the 2 star category in sometimes poor locations like the Royal in Bangkok. Nothing wrong with this if this is what you want. Kimbo gives good advice and since I have not been to Vietnam I cannot help you much on that end. Cambodia yes as I have been several times and there is more to the country than Siem Reap with its world known temples. I disagree with using a tiuk-tuk especially in the july/august heat. An air-conditioned car is best although it will cost more and perhaps a guide. In Phnom Penh a guide is not necessary but again car beats the tuk-tuk for comfort.
You say july/august, does that mean you will be traveling for two months. If so you could spend 2 weeks in Vietnam, say 11/2 in Cambodia (see more of the country other than SR and PP), visit Luang Prabang in Laos, explore northern Thailand, spend at least 5 days in Bangkok, maybe a beach in the south, visit Malaysia or Burma or both. On the way there or back you could also stop in Hong Kong. These are just examples of thing to do and see.

What type of hotels were you thinking of budget, mid-range or deluxe and price range. Two rooms or 3 in a room.

Bangkok Air has the Discovery Pass as mentioned and you have to buy a minimum of 3 flights. This pass is also good on Siem Reap and Laos airlines. International flights are $100 and domestic 60. Add $22for fuel charges etc for each flight plus airport taxes. The pass has to be obtained outside of Thailand either from a travel agent or by calling a BA office.

Let us know some more information and we may be able to assist you further with your plans