You'll likely spend much of your trip indoors, but if you're planning to venture beyond the casino, taking a look at local weather data can help you. The hottest months are usually April and March with an average temp of 84°F, while the coldest months are January and December with an average of 75°F. The rainiest months in Vientiane are August, July, September, and June, with each month seeing an average of 16 inches of rainfall.