Fears are growing for the safety of the jailed blogger, Nguyen Ngoc Nhu Quynh, because of poor conditions in prison and hostility from her fellow prisoners.
She is reported to have warned her mother that she is close to breaking point and fears she could die in prison if the situation does not improve.
Quynh, who wrote a blog under the name Me Nam or Mother Mushroom, is serving a ten year sentence after being convicted of conducting propaganda against the state.
Her supporters fear that the authorities are trying to break her physically and psychologically because of her high profile at home and abroad as a symbol of free speech.
Speaking after her latest visit, her mother, Nguyen Thi Tuyet Lan, said that Quynh could no longer bear the mental torture she was being subjected to.
She said she had been moved from a cell with virtually no ventilation to one with two other prisoners, and one of the cell mates subjected her to continuous torrents of abuse.
The authorities have been known in the past to co-opt ordinary prisoners to make lives miserable for political detainees.
Quynh was moved earlier this year from a prison within reach of her home in Nha Trang to one hundreds of kilometres away in the northern province of Thanh Hoa.
The transfer makes it hard for her mother and young son and daughter to visit.
The authorities have been accused in the past of attempting to break the spirit of jailed government critics. In a number of cases they have attempted to make conditions intolerable to force detainees to accept exile abroad.
The human rights lawyer, Nguyen Van Dai, is the latest prominent prisoner to agree to leave the country. He was flown to Germany earlier this month.
The Communist authorities believe they can reduce the impact of their most influential critics by arranging for political asylum abroad, usually in Europe or the United States.
Quynh has attempted to fight back against her treatment, at times refusing to eat in protest against the poor quality of food.
Her mother is increasingly worried about her health and her state of mind.