Ho Van Thanh, 82, and his son, Ho Van Lang, 42, emerged bedraggled from the remote mountainous region on Wednesday in an emaciated condition, wearing loincloths made from tree bark.
The former communist soldier Thanh ran away with his then two-year-old son Lang in 1972 from a communist village in central Quang Ngai province.
He was grief-stricken following the death of his mother and two of his other children in an American bombing, local official Hoang Anh Ngoc told AFP.
"The son is afraid of the crowds. He will not talk to strangers... but he talks inside their family," Ngoc said, adding authorities will spend about $2,300 to build the men a house near their relatives.
While the older man is frail and under medical supervision, Ngoc said officials still have to "keep our eyes" on Lang to prevent him escaping back to the forest where the pair lived in a hut five metres off the forest floor, reportedly surviving on foraged fruits and corn they cultivated.
Television footage showed authorities apparently taking the pair against their will from their forest hut several hours' walk from their home village, with the frail old man carried in a hammock by local people while the son was pictured with restraints around his hands.
The pair were "were dressed only in loincloths made of tree bark", Tuoi Tre newspaper said Friday, adding they were found with several handmade tools including axes made from war-era shells.
The pair can speak little of their ethnic Kor language.
The men were first brought back home by a younger son in 2004, Dan Tri online newspaper reported Friday quoting local authorities, but they could not adapt to living in the village and returned to their forest home.
The pair "preferred their independent life to that of the traditional Vietnamese family", the report said.
The younger son visited them once every year, providing some necessities.
But they were recently spotted by local residents and reported to the authorities who drew them from the forest on Wednesday.
After being captured Lang reportedly "chewed betel nut and smoked continuously, glancing at everybody around him with a dull look", according to the Tuoi Tre report.