Yes, you can pay with euros in Laos. However, the exchange rate will almost certainly be lower than if you paid in kip. The kip is Laos' official currency. However, euros are often accepted in tourist regions. You may obtain a better exchange rate if you convert your euros for a kip at a bank or money exchange bureau. However, most stores and restaurants accept euros.
Here are some places where you can pay with euros in Laos:
It is always a good idea to ask before you pay with euros, as some businesses may not accept them.