I think Aqua Expeditions is the leader in the market.... but more pax that what DavidO suggested
https://www.aquaexpeditions.com/luxu...s/aqua-mekong/
Heritage Line is owned by a large DMC operator in the region who have excellent pricing on the ship when blended with their other products.
Sometimes you are stuck a bit based on your flight dates, but if OP chose the ship first the rest it will make planning easier