Please be aware that Bangkok has multiple airports and that these results are for all airports serving the city.
The best month to fly between Bangkok and Vientiane is February and the most expensive month to fly is March. (Average prices, based on 430 datapoints.)
The cheapest airline flying from Bangkok to Vientiane is Thai AirAsia. They are 68% cheaper than Thai Airways. (Average prices, based on 434 datapoints.)