Most 8-day cruises include two countries (Vietnam and Cambodia) in the itinerary. The tour usually starts in Saigon and ends in Phnom Penh (upstream). The downstream cruise has mostly the same attractions but in reverse.
There is also the Heritage Line Anouvong Cruise, which visits only 1 country (Laos) and travels from Huay Xai to Vientiane and vice versa.