Dao Mau has no central registering body and is practised alongside, and often within, Vietnamese Buddhism and folk religion generally, so Vietnam's census and outside demographic surveys have never separated it out as its own counted category.
What would resolve this A survey instrument that asks specifically about Len Dong participation or Mother Goddess devotion, distinct from general Buddhist or folk religious identification, applied at national scale in Vietnam.
Sociology of religionUNESCO Intergovernmental Committee for the Safeguarding of the Intangible Cultural Heritage, Practices Related to the Viet Beliefs in the Mother Goddesses of Three Realms, UNESCO Representative List of the Intangible Cultural Heritage of Humanity