I have not been on it yet, but the best cruises between the Golden Triangle and Luang Prabang, and elsewhere in that vicinity, are operated by Mekong Kingdoms, which is owned by Minor International (Anantara, Four Seasons Thailand, etc...).

Incidentally, I have been looking into Rosewood Phnom Penh, and noticed that they offer a package called “Sense of Mekong” in conjunction with a cruise on a vessel called Aqua Mekong, which looks very nice. Presumably journeys on Aqua Mekong are also bookable independently, and perhaps there are different itineraries to choose from. Sense-of-Mekong