Indeed, welcome to the Outdoor / Adv. Travel forum.
In short, it will not really be possible to see each of these countries in two weeks. Transportation is slow in SE Asia and you will have most of your time in slow moving buses and border crossings I'm afraid.
Realistically with two weeks you are going to be limited to one country like Thailand and then perhaps a side trip to Ankor What in Cambodia. Alternatively, there is plenty to see in Malaysia as well that can take up much of your time. I would not rush away without seeing Cameroon Islands, Penang (perhaps Melaka and KL as well) and perhaps the Perentian islands before you even enter Thailand. Once in Thailand there is 6-8 months with of locations to visit. It is hard to leave. I would spend no less than 3-4 days at a beach resort (Ko Phi Phi, Ko Samui, Ko Samed, Ko Tai or one of many others) and no less than 3-4 days in Bangkok. I would then suggest going to Chang Mai for no less than 3 days and it takes 12 hours to get there from Bangkok. As you can see, your 2 weeks will be finished before you can really even begin to get a feel for Thailand.
Another alternative if you have a big interest in Vietnam is to fly there directly from KL. I would then suggest concentrating on either the North or the South as two weeks is not enough time to see both without being in buses about 1/2 the time.
As such, I'd suggest getting a map, doing a bit of research online and with reading up on lots of forum posts on Tripadvisor and the Lonely Plant GumTree forums so that you can understand what is really possible in two weeks. Then draft a rough plan, post it here and we can help critique for you and offer our thoughts and suggestions.
Best of luck with your research and hope you enjoy your trip planning.