Vietnam, Laos and Cambodia will all give you a similar experience regards climate, landscape, people. I would identify specific areas/ sights and select them across the region eg temples Cambodia / war museums Vietnam / jungle trek Laos / beach break Vietnam / old colonial city Cambodia etc
Lonely Planet guides usually give a 'not to be missed' list for each country and i'd pick and choose from there then schedule.
Top tip for Asia; allow extra time. Things don't go to plan or you unexpectedly come across something amazing and it's not worth rushing about.
Thailand and Malaysia are different because you can choose between undeveloped and very developed areas. Bangkok is an experience but rather squalid IMHO.
Personally, when I go to Malaysia i head to Sabah which is in North Borneo; nice civilised cities such as Sandakhan but with amazing wilderness, unspoilt beaches, islands and reefs (some of the best diving in the world) and unique (endangered) wildlife eg Orang Utan and Probosis Monkey
Whatever, you have a great time and remember to take your malaria tablets
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