There are a few different companies that offer similar cruises up the Mekong, but we had sailed with Viking River Cruises in the past and were happy with them. I booked the April sailing, the last of the season because we could not sail earlier. This meant that we had to deal with the summer heat, which made the trip uncomfortable at times, but still well worthwhile.
We took the 3 day Bangkok pre cruise extension and the 2 day Halong Bay post cruise extension. I would recommend both if you have the time, since the 12 hour difference in time from North America causes some jet lag that needs to be overcome. Both places are worth a visit if you travel that far.
We enjoyed the company of 52 other passengers, most were about the same age group of active 60 and 70 year old travelers. We got along well. The ship (boat) is very nice, built for Viking by Pandow, and is named Pandow Tonle, after the Cambodian Lake of that name. Cambodian regulations did not allow Viking to put their name on a boat until they have done business there for many years. Perhaps they will design and built a new vessel in the future, but this one was just fine, and described in detail in previous reviews.