Getting to Ang Nam Ngum by public transport is easy enough. From Vientiane, four government buses depart daily from the bus station near the morning market for Thalat (there are also more frequent sawngthaews here, leaving from the stand in front of the bus station). From Thalat, you can get a shared tuk-tuk to the reservoir (30 min; $5) or the regular tuk-tuk shuttle service to Na Nam, near the dam on the western shore of the reservoir, and the most logical base for independent travellers to explore the lake by chartering a taxi ($25 return). If you go for this option, you might consider making a scenic detour en route along the quieter Route 10 via Ban Keun. Southbound sawngthaews from Vang Vieng pass right by the town of Tha Hua at the northern end of the reservoir, or you can switch vehicles at the Phonhong junction and cut in to Thalat and Na Nam. If you want to use the lake as an alternative route from Vientiane north to Vang Vieng, charter a boat from Na Nam for the five-hour trip to Tha Hua and then continue by road to Vang Vieng.
Travel companies offer one- to three-day package tours to the reservoir out of Vang Vieng, which combine hiking, boating and camping, but expect to pay considerably more for the convenience. You can usually arrange a private boat trip on the reservoir by asking around in Na Nam. Guesthouses and hotels mentioned here should also be able to point you in the direction of the best hiking trails.