I think your itinerary is fine. Some more suggestions from my experiences:
In Vientain: visit Talat Sao market (the biggest one) where they well a lot of stuff, especially silver (but consider its quality if you have deep understanding of jewelry. Walking along Mekong river bank in the afternoon, visit night market there and then spend your evening at Nam Phou fountain square, so great with live music and reasonably priced drinks, next to Kop Chai Deu - a famous restaurant (as one local told me, its name means "thank you"). Besides, in the day time, let's visit Wat Si Muang (Mother temple as the local people call). There is a special stone (about 10-20kg) and the local people believe that anyone who can move it over their heads 3 times and feel it's light, their wish will come true (I did it but failed!), visit Pha That Luang and National museum .
In Vang Vieng: travel by hot air-balloon (2 years ago it costed 80$/person/45min) in the early morning (fly on the sky at sunrise time) or at sunset time (exciting!), visit Water cave and do tubing there (adventurous feeling) and do kayaking then, visit Blue lagoon (very nice). In the evening, stop by local bar, restaurant to enjoy local food and drink, delicious and much cheaper than in Luang Prabang. In the restaurant, you can sit or even lie because of its special arrangement (chairs and tables are nearly the same as bed!)
In Luang Prabang: you can spend a half of day to walk around the city and visit different temples, the must-see one is Wat Xieng Thong. Also, you can stop by a house (quite near the temple) where they sell a lot of special paper products (books, cards, lantern, envelop....) although the price is a little more expensive than in the night market). The 1st day, sitting at Mekong river bank at sunset and have dinner then . The 2nd day, go up to Phou Si mount to see sunset (excellent view and a lot of people), so you should go there early to have a good position. In the evening, visit night market where you can buy a lot of nice fabrics at reasonable price and a number of things, enjoy local snacks, drinks and cheap buffet as well.... In day time, walking around the city, stop by different shops or restaurants will give you great experience. Besides, don't skip Tad Sea waterfall (where you can do zip lining) and some trekking to local village, and Kuang Si waterfall where you can do swimming and see black bears. One more place to visit is Pak Ou cave (a thousand of Buddhist statues).
Gluck & hope it useful!