Have a look at Indochina Junk. My partner and I did the two night cruise last September and it was fantastic. We would have like an extra night or two. Cabins are lovely and spacious and the views are spectacular. We booked the 22 birth Junk, can't remember the name, and only two other couples aboard which made it all the better. Had so much fun together, lots of fine food and drink. It really was a highlight of our four weeks away. I would do it all again and with the same company. We booked online from Australia.
Happy travels.