Not a QP, but I speak Thai. I did some self study in Lao out of a book and subsequently made a score on a Lao test. If you can't find Lao materials then you should learn as much Thai as possible. It will help. The languages come from the same origins. I don't think it's a stretch to call them dialects of the same langauge.
Concerning both, tones are everything for understanding and making yourself understood. Grammar is basically the same. Same with vocab, although pronunciations of same words may be different. Any vocabulary you can pick up and your demonstration of a desire to learn the language will make your trip much more rewarding. I don't know how true it is, but I've heard that the Lao listen/watch Thai radio and TV more than Lao stations. Bottom line, some Thai is better than nothing. Luang Prabang, I believe, was the French colonial capital of Laos. Therefore, some French might be helpful. However, use of French might have some cultural stigma attached...don't know. Communicating is better than not communicating though, at least in my book.
The Lao greeting is "Sabai Dee". Use it frequently. As an aside, in Thai it is used to tell someone you're comfortable or feeling well.
Speaking of books. For general prep, the first order of business I recommend is to pick up a Lonely Planet Guide for Laos. Amazon has them. If I could only have one info source prior to travel anywhere, the applicable LP guide is the one I would choose. Most of them have some basic lingual phrases included.
You've probably been to this site already:
HTML Code:
http://travel.state.gov/travel/cis_pa_tw/cis/cis_946.html
. It's a good starting point to some of the other questions
When I got my shots, there were several that came in a series. You need to get started ASAP and I'm not sure you'll have time to finish them. You'll need to confer with a medical professional on the implications of not finishing a shot series. You also need to check into the requirement for malaria prophylaxis, too.
Kor hai chok dee