The Phra Bang is a standing Buddha image about eighty centimetres high, cast in an alloy with a high gold content, with both palms raised in the gesture of dispelling fear. In the Lao account it came from Ceylon to Angkor and from Angkor to Fa Ngum, arriving with a delegation of monks who brought the Pali canon and the ordination lineage with them. It became the object on which the kingdom's fortune was held to depend, was carried off twice by Siamese armies and returned twice, and gave the capital the name it still bears.
The account is not contemporary. The Lao chronicles were compiled from the sixteenth century onward, and their treatment of the fourteenth is a mixture of record, dynastic claim and hagiography. Historians of Laos accordingly separate what the chronicles say happened from what the chronicles are doing: a founding king who receives the religion, the canon and a palladium from the greatest court of the region is a story that legitimates a dynasty, and it is told in the same terms in several neighbouring kingdoms. The image itself has been dated by art historians on stylistic grounds to a period later than Fa Ngum, which if correct places its arrival after the reign the chronicles assign it to.
None of that makes the establishment fictional. Theravada Buddhism of the Sinhalese ordination line did spread through mainland Southeast Asia in exactly this period and by exactly this mechanism, courts importing monks and texts from a prestigious centre. What is uncertain is the timetable and the personal role of the founding king, not the process.