Hi snakehips
I spend three months at the beginning of this year in Vietn., Cambodia, Laos and a bit of Thailand. That's what I can consider you.
1. try to fly as few as possible. Flying might be a little bit faster but you miss a big part of a country or culture. So you are the "tipical" tourist hopping in and out of a country but you don't get the insight of it.
Five weeks for three country seems to me quite a lot, especially if you want to stay in Thailand for a while as well. That's what I would do:
Book a fly in BKK (go there, see some from BKK and book a ticket there one/two days in advance) to Saigon. You can spend there 3 days easily (tunnels from Cu Chi, pagodas, walking around, ice coffees!!). If you like beaches go to Mui Ne with big sanddunes (around 5hrs with a bus (Sinh Cafe) leaving in the middle of the tourist area in Saigon, has noting to do with a hotel, take a jacket on board and a tape to close the freezing aircon!). Mui Ne is known for surfing and must be really pretty. I should have gone there...!
Then go back to Saigon and take a tour to the Mekong Delta. There are tours that lead you to Cambodia, directly to Phnom Penh. This city is also very interesting. good for 3-5 days. Go to the Banana top guesthouse at street pasteur 51.
The Siem Reap for 3 days, go to the lake (Ton le Sap) for one day and take a bus to Cambodia. The street is no problem anymore. It must have been hell some years before but in February the biggest part was great, some parts were under construction. Might be different in August though when it rains.
I went to Ko Chang directly from SR (around 8hrs on a very good bus and the mini van). Ko Chang is a nice island, not too big, some tourist resorts but also very laid back ones (ex. KP Huts). Rent a motorbike there, check the island, go snorkeling and have a suit/dress made there, in a relaxed atmosphere.
So:
BKK 3-5days
Saigon 5 days
Mui Ne 5 days
Saigon 1 day
Mekong 3 days
PP 4-5 days
SR 4-5 days
Ko Chang 6 days
BKK 3 days (direct bus from Ko Chang, around 6hrs)
total: 36 days (5 weeks)
well, hope you can dicide on a tour. :-) Have fun.
kindly
Pascal from Switzerland