In Thailand, the currency is the Baht (THB) and in Laos the Kip (LAK).
1 € is about 50 THB and 12 000 LAK.
1 $ is about 40 Baht and 9 000 LAK.
The Dollar is used in Laos, so you can take some with you to pay.
The Baht is widely used, so you can take Euros and Dollars with you, and your guide will show where you can change it. Or you can change it at the airport on arrival.
You can use Visa or Mastercard credit cards at ATMs (If you have two credit cards, bring both, in case one does not work.) in NongKhai and Vientiane. Make sure to notify your bank/credit card companies that you will be using the cards in SE Asia. Bring some cash too, because the ATM machines charge quite a high fee.