Jill Mary made the call to Minister of State Hugo Swire six months after 42-year-old Jamie Taggart went missing while on a plant-documenting expedition in a mountainous region in the north of the country.
Mr Taggart runs the world-renowned Linn Botanic Garden at Cove, in Argyll and Bute, that his father, Dr Jim Taggart, founded.
The community there has rallied to help raise funds for extensive searches of the area that have been carried out by Vietnamese national park authorities, as well has privately hired local search parties.
Mr Taggart's mother has also been planning to hire a private detective to carry out further inquiries in Vietnam.
She said on a social media site that she still believes her son is alive.
Mrs Mary said: "Jamie is missing in Vietnam and has been for six months. We do not know what has happened to him.
"He is the first thing I think of when I wake and the last before I sleep, when I can. All day I puzzle over what has happened to him - has he has been abducted, injured or killed or chosen to disappear and lead a different life?"
Mr Taggart's friends and family continue to raise money to fund the investigation.
His mother said: "I also have been able to add to the funds for searching for Jamie. Contributions have been sent to the fund from myself, my friends and contacts."
She wrote to Mr Swire: "Although there is no more news, I am glad of the information that the British Embassy in Beijing has made inquiries with the Chinese authorities to confirm that he is not being detained by them.
"It is despite intensive searches have revealed no sign of him, and none have been reported, I feel that he is still alive, but is unable to make us aware of where he is, either for involuntary or personal reasons.
"Because it is possible that he may be a prisoner I need to know that all inquiries have been made to eliminate this possibility."
She added that staff at the British Consul in Vietnam had informed her there were inquiries made to the country's authorities with similar results.
Mrs Mary added: "I just feel there must be people who do know what has happened to him."
Mr Taggart senior has also been pressing the UK Government for more information about the disappearance, as he tries to piece together the events since his son left his boarding house in the town of Sa Pa to search for rare plants on October 31.
He left the town on a motorbike taxi heading towards the Hoang Lien National Park.
A spokesman for the Foreign Office said yesterday that it cannot discuss details of individual cases. He added: "We continue to provide consular assistance to their family at this difficult time."
A Foreign Office official was earlier quoted as saying the family had been provided with all information that has been passed by the Vietnamese authorities.