Viengchampa is a tour operator based in Vientiane with links all over the country. They organised a trip I took in southern Laos in September 2008 very efficiently. I only had a few days to spare and wanted to get a reasonable overview of the southern provinces without the hassle of organising it myself and Viengchampa were great. They use local guides who really know the area and all of them spoke good English and were very competent. Everything happened pretty much when they said it would and they were flexible enough to allow stops whenever a place looked interesting. Highly recommended.
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