Wattay International Airport sits just a few kilometers from the center of Vientiane. The signal here is strong. If you set up the eSIM before your flight, it will pick up Lao Telecom on its own once you switch on the line, usually within a minute or two of landing.
Vientiane is the easiest place in the country to stay connected. Along the Mekong riverfront, around Patuxai, and near the night market you get steady 4G. This is where you will call a Grab car, check your hotel booking, and send your first photos home. A small plan covers a day or two here with no trouble. The 1 GB for 7 days plan at $2.05 is plenty if Vientiane is just your entry point.