Maybe the small figure represents “pure” Buddhists, while most Viets practice basically a combination or hybrid belief system of folk religion and Buddhism, among other beliefs.
Most VietNamese (in my experience) practiced Folk Religion.
It is derived from a blend of Confucianism and The Tao.
It tends to be animistic and have a lot of Ancestral worship
both of person (The Ancestors themselves) and place (the
village, or plot of farmland.)
It is also quite non-rigid and non-dogmatic.
It is not uncommon to enter a home and see a “family altar” display that
includes elements of Christianity, Cao Dai, a Buddha and pictures of the
aforementioned ancestors.
Depending on how a survey was worded, polling for sect might well yield
a total in excess of 100%.
Seeing a display of a Christian Cross or a statue of Buddha
in these settings does not make them a Catholic, Christian
or Buddhist.
So essentially I would agree with what you said.
you are late to the ball.