Well I wouldn't bother about diving in Cambodia if you are doing Thailand and Indo. For Cambodia, I'd concentrate on topside. Angkor Wat/Siem Reap I'm sure you'll be going to, but the smaller places like Sambor are also very nice but harder to get to. I liked Battambang a lot and a great way to see it is via the Mekong.
You can also try to get the Irrawaddy dolphins in Krati (Sp.?) but I hear there are fewer dolphins now than before. I'd check with WWF site for updates. I can't remember where but there are places where you can wade with giant river rays. I'd bring a housing and fisheye for that

From Angkor Wat to the Khmer massacres, there's so much to see and learn.
Vietnam is also pretty fished out so it's mostly macro, and nothing you'd not see with much more diversity in Indo. I wouldn't bother with scuba with either country. There's so much recent and ancient historical places to go to and the diving is absolutely crap compared to Indo.