The most convenient way to travel in Laos is by mini bus. Just choose one of the small companies or families who seem more or less reliable. I payed around 5 euro from Vientiane to Vang Vieng (3-4 hours) and around 9 euro from Vang Vieng to Luang Prabang (6-7 hours). Might be more expensive now. Couchsurfing is not very popular there. So I stayed in hostels. Less than 5 euro per night. The food is even cheaper.
Vang Vieng: beautiful landscape and many outdoor activities (climb, river rafting, canoe, caves). There I spend my birthday. I went tubing on the river (some get really drunk since you can stop along the river in some bars), played volleyball, had fun in the mud, jumped in the water, and danced in the rain.
Luang Prabang: I loved the Kuang Xi Waterfall (3.50 euro). If you walk up the hill you won’t see many tourists anymore. The Mount Phousi by sunrise and sunrise is a nice experience. Same for the night market, bamboo bridge, and the morning market. Luang Prabang has great bars with live music.