I am a solo traveller to Vietnam in February and am looking for recommendations for a cruise company offering 2 or 3 nights on Halong Bay. I would like a luxury level.

Check Indochina Junk Company. They sail to Bai Tu Long Bay, near Halkong Bay but fewer other ships. No others were there when I was there last October.
Like you, I went "solo" -- on an overnight trip. Great food, stops, and fellow travelers. They have two-night cruises available also.

Let me second CaliNurse's good recommendation for an Indochina Junk trip in Bai Tu Long Bay.

After reading about how crowded and increasingly polluted Halong Bay has become, we decided to go to Bai Tu Long Bay, and were very happy with that spectacular but uncrowded location. We were usually the only vessel other than the occasional tiny Vietnamese fishing boat

We decided to make it even more special by chartering one of their private one-bedroom junks for an overnight cruise, and it was just wonderful. They call these 2-passenger junks a Princess junk. The food was delicious (well, except for the breakfast and coffee!), and we loved being able to have the captain change course when we spotted something intriguing.

The only place we saw a lot of other junks/people was at the "floating fishing village", which we asked the captain to leave as soon as we saw it. And after we'd moved on, our on-board guide sheepishly admitted that I was correct when I suggested it was not a real working fishing village, but a subsidized tourist trap.

I have no doubt that the larger (and less costly) junks which hold more people are also delightful.

Thankyou both for your replies. I have checked out Indochina Junk. Looks good.
Does anyone else in Fodors world have another they liked?