buddhism ?

Probably Hinduism because there's lots of ancient Hindu temples in Laos such as Wat Phu, and there's a Buddha Park in Vientiane with old Hindu and Buddhist statues. Buddhism and Hinduism are a similar religion.

Actually, the indigenous peoples of Laos would have practiced Animism. Buddhism was introduced to Laos by Fa Ngum from Khmer Empire, where he established it as the official religion of his new kingdom.

Obviously there are vast differences between the Buddhism of that time and today, but its Laos' cultural mix of Animism with Buddhism that defined the Lao cultural identity. Generally, Lao Therevada Buddhism has distinctions, from say Burmese Therevada Buddhism because of those influences.