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The easiest is probably to fly into Hanoi. Since your time is very very very very limited you should.only see the north of Vietnam, cross to Laos and only see Luang Prabang and fly to Phnom Penh and see Battambang and Siem Reap. From Siem Reap you could probably take a connecting flight again via Vietnam or Thailand o get back to Europe.
With 4 weeks best to try for Da Nang on central coast Viet and work your way south to HCMC, then head for PP and Siem Reap and Cambodia Islands or Phu Quoc and return to HCMC for exit home.
Drop Laos, you'll lose to many days to get in and out by land.
Four weeks... I would choose either all of Vietnam or Laos and Cambodia, otherwise you will be running around too fast and not absorb and really enjoy anywhere!