Sorry, I can only write in English.
Yes, renting a bike locally is the way to go. For example, in Vietnam I would recommend QT up in the North: https://qtmotorbikesandtours.com.vn/...iang-cao-bang/ but I think you can rent a Honda XR150 from other agencies down south too. It is the best option for that part of the world - it's tall and offroad-capable, but small and light. It's slow by European standards, but still faster than the moped traffic around it.
I think you can cross into Laos and Cambodia on bikes with Vietnamese plates and vice versa. But with only 15 days, you might as well fly or take a night bus between destinations, and rent scooters when you get there. You can rent a scooter anywhere in SE Asia, any hostel will tell you where.
Theoretically there are requirements about getting temporary local licenses, etc. - in practice nobody cares. Vietnamese *traffic* police don't stop tourists.
Do your research about visas: EU citizens can get a visa-on-arrival in both Laos and Vietnam (I have not been to Cambodia so not sure), but you need dollars for Laos, and for Vietnam you also need to apply for it in advance through a tour agency. And you can only get them at airports, not at land crossings.
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