You been to any of them before?
Laos is wicked, 5 weeks there would be good. I have only seen a limited amount of the country (Viettiane, Vang Vieng and Luang Prabang) but I loved it. People are incredibly friendly, food is fantastic, lots of scenery, nature and cool activities. Vang Vieng alone has 'tubing' which is the best thing I've ever done plus you can go swim in the blue lagoon, explore caves, kayaking, biking etc. Luang Prabang is a cool place with a very cosmopolitan and European vibe, lots of nice cafes, coffee shops and bakeries as well as an incredibe night market. The temple up on the hill offers great views of the city. Nearbye there's waterfalls, elepheant trekking and so on.
Vietnam is OK. Halong Bay in the north is nice, Hanoi is a horrible hectic hell hole but the Army Museum is cool and Hoi Chi Minhs masoleum is there, Hue has the DMZ tunnels and not much else - pretty boring, Hoi An has some incredible street food and is kind of pretty plus good place to get a suit / clothes made but not exciting, Nha Trang is a cool beach town which is a nice place to spend some time relaxing -spent the festive period here and had a great time drinking passionfruit beer all day by the pool at the Louisiana Brew House where they make their own beer then there is Saigon which is so incredibly busy and chaotic that it should be avoided at all costs, the only bonus is that nearbye is the Cu Chi tunnels and Mekong Delta.