Actually, there is a store in Wattay International Airport for sim card buy I reached after 9 pm. It was closed. When I checked with the taxi guy he told its open on the morning.
Every convenience shops sell sim card. I bought it from a mobile shop ( a lot are there).
Buy from a mobile shop = You can check and the guy explains the plan
convenience shops = Guy will be always busy with other customers.
I did small research for buying Sim card and found
There are data cards + Data $ call cards. So select depending on your need and use.
BEEline / Lao Telecom / Unitel serves very well in Vientiane area. But travelling other areas not sure.
I bought Lao Telecom Data Sim and got LTE(4g) in most areas even while travelling in Bus. WOrled very welll in Southern areas too.
Paid 150000 (16$ ) for 15 GB data. There are smaller plans of course,
Sim validity is for 6 months and they didn’t ask for passport copy (Identity proof).
Worked well for me.
And better to buy outside since airport prices are more expensive.