Minor also has Mekong Kingoms, which is absolutely ideal for connecting Golden Triangle with Luang Prabang, and other cruises in that vicinity. It is a private charter, so perhaps they could do other itineraries upon request. Their vessel called Gypsy, I believe, is ideal for a couple or a family/with friends.

There must be other suitable vessels for charter between Siem Reap and Phnom Penh, Saigon etc...