The Zao people are an Asian ethnic group from the mountainous regions of China, Vietnam, Laos, and Thailand. The red zao are also one of the sub-groups of the Dzao ethnicity in southern China. Red zao groups began a gradual southward migration in the 18th century due to political unrest and to find more arable land. Zao people minority people have villages all around North of Vietnam.
The Red Zao are the 4rd largest minority in Vietnam with nearly 500,000 people. In the Sapa region there are about 35,000 Red Zao residing in the surrounding many villages and they account for about 30% of the area population. Most reside at an altitude between 1,600 and 1,700 meters above sea level in this stunningly beautiful region they call "the city above the clouds". The Zao started migrating to Vietnam from China last 300 years.
We are still not sure about the difference ...
Without the H or with it, the DAO prononciation is the same than ZAO
There is 4,030,000 RED DAO results and 612,000 ZAO results on a Google search.