The rescue at the bridge construction site has been going on for two days
Reports say Ly Hao Nam was searching for scrap metal when he fell into the hollow concrete pile in southern Dong Thap province on New Year's Eve.
Teams have battled since then to free him, pumping oxygen into the 35m-deep support pillar to help him breathe.
He was heard crying for help two days ago but gave no response when a camera was lowered down the hole on Monday.
"We are trying our best. We cannot tell the boy's condition yet," one rescuer at the bridge construction site in the Mekong delta told AFP news agency.
Officials have been amazed that anyone could fall down the shaft in the pillar, which measures just 25cm (10in) in diameter.
Photos from the scene have shown the boy's parents watching anxiously as rescuers try to reach their son.
Emergency teams have tried to lift the pile with cranes and excavators, but so far with no success.
They have also softened the soil around the pillar to try to free it, but that has made it tilt and complicated rescue efforts, Reuters news agency reported.
-- Courtesy of BBC News