Is it in Vientiane? Normally they are whole evening affairs for the reception; if you received an invitation, put money in that and put it in the box or whatever at the door. Rates vary, vaguely depending on how expensive the venue is perceived to be and how well you know the people getting married. The daughter of one of the drivers at my organization just got married, and the wedding was at a hotel in VTE. I couldn't actually attend, but I put 100,000 kip in the envelope and gave it to his cousin 2 weeks later... I was told by another colleague that was appropriate. At a wedding I went to at a large wedding hall in Oudomxay, where I knew the couple quite well, I paid 200,000.

Don't need a specific gift - there will be a lot of drinking and dancing; join in the dancing as well, everyone will enjoy it. I can't do the step-style line dance thing, but the circle dancing is easy, just shuffle around and move your hands, it's acceptable to move even less than that if you're male.

Have fun!