'We are hoping that the high commissioner (Navanethem Pillay) will put pressure on the government to reconsider more closely our situation and to find solutions,' the monk told AFP in Geneva.
The UNHCR official is to take up the issue with Vietnam's ambassador to the United Nations in Geneva, Trung Hai added.
Thich Nhat Hanh, 83, has been a key figure in the growth of Buddhism in the West, but his opposition to the Vietnam war led to a life in exile in France where he established the Plum Village retreat in the Dordogne region.
He was allowed to return to his homeland for a visit in 2006.
Trung Hai, who has been living several months in France, was especially worried for fellow followers of Thich Nhat Hanh inside Vietnam who are facing what he called 'concrete threats' and 'harassment' from the authorities.
Some 100 monks were forced out of their monastery in Bat Nha, near Bao Loc city, in September, he said, while other followers have taken refugee in another nearby monastery where they are coming under 'strong police pressure'.
'For the authorities, we are illegal,' he said.