Essential destinations in Vietnam, Cambodia and Laos?

I would focus on Vietnam and Cambodia. Vietnam has a lot to offer and a very well running tourism industry. Two weeks is highly recommended there, as you should see both north, south and central Vietnam. In Cambodia you have Angkor Wat, which is probably the single biggest sight in all Asia. Cambodia is also a very welcoming country, that is so easy to fall in love with. In Vietnam, Ha Long Bay is the biggest sight. It's important to spent a couple of days exploring Halong though, as it gets crowded if you only visit the main places. I have a blogpost, where I explain a little more in detail, how to avoid the pitfalls, when visitng Halong Bay: www.travellingclaus.com/halong-bay

You can do Vietnam only in three weeks (North to South of VN or vice versa) or Vietnam and 3 or 4 full days in Cambodia especially Angkor Wat.