The route can be done in either direction, both are just as good.
As for advice as to what to see, I suggest you get hold of copies of the Lonely Planet for Laos and Thailand. Try your local library if you don't want to buy them.
Have fun.
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As long as you are that far up in Laos at at Huay Xai, I suggest that you do a loop by bus through upper Laos through Luang Nam Tha, Muang Sing, Nong Khiauw and then a boat down to Luang Prabang. Great scenery and sights
Vang Vieng, is many peoples' opinion including mine, is an tourist-ridden pit with lousy food and not much to recommend it.
(to be continued)
