The census figure that shows most Vietnamese with no religion is an artefact of what the form counts. What is actually practised is largely unaffiliated, has no membership and no clergy, and takes place in three settings.
The ancestor altar
Almost every household keeps one, usually on the highest shelf in the main room, with photographs, incense and offerings. Death anniversaries rather than birthdays are the family calendar, and the eldest son inherits the duty of keeping them. Nothing about this requires belonging to anything, which is precisely why it does not register as religion on a survey form, and it is the most widely practised religious activity in the country.
The dinh
The communal house of a village holds the cult of its tutelary spirit, often a founder, a general or a local figure whose status was confirmed by imperial decree. The dinh was the civic and religious centre together, was heavily damaged by war and by campaigns against superstition, and has been rebuilt widely since the 1990s.
Dao Mau
The mother goddess religion venerates a set of goddesses across three or four realms, and its central practice is len dong, a mediumship rite in which a spirit is received by a medium in elaborate costume, with music and song. It was suppressed as superstition for decades. In 2016 the practices associated with the Viet mother goddesses were inscribed on the UNESCO intangible heritage list, which has changed its public standing considerably and has also brought complaints from practitioners about the commercialisation that followed.