The currency of Laos is the Kip which have a quite low value and changing currency foreign currency always ends in being handed over a very large lump of Kip. (Which also makes the currency a great present for family and friends.) In most of the tourist areas you can pay with Thai Baht as well as US-dollar but you get the best price using Kip.
It is illiegal for unlicencensed person to do any banking service so change your money at any of the larger banks to be sure. Some of larger Thai banks have offices in Vientiane. This also includes changing notes into smaller amounts, some shopekeepers will do this without you buying anything and they are then breaking the law. This is not often checked but you and the shopkeeper can get into trouble. If you need to get smaller notes buy something or if you can't change the money discreatly.