The government of Vietnam exercises near absolute control over all religions. The Unified Buddhist Church of Vietnam has been banned since the early 1980s when it refused to join the state-sanctioned Vietnam Buddhist Church. I recall reading a Human Rights Watch report last year about how the government forcibly stage managed the funeral of Thich Huyen Quang, the previous head of UBCV, after keeping him in prison, under house arrest, or in internal exile for most of the last three decades of his long life. Evidently Thich Quang Do, current head of the UBCV, is himself under house arrest in Ho Chi Minh City even now.
I don’t know very much about Buddhism in Vietnam at this time, but I do know that totalitarian governments everywhere fear religious challenges to their power. The greatest threat to the Burmese army is certainly the Burmese Sangha, who dared to chant the Metta Sutta with guns trained on them.
Cracking down on Thich Nhat Hanh and his followers might just prove that his work for human rights and community development is proving too successful for the Communist party to tolerate. If the monks and nuns who look to Thich Nhat Hanh as their leader are being beaten and humiliated by police and mobs, does it make sense to blame them rather than see them as victims whose human rights are being violated? Perhaps the rulers of Vietnam regret loosening the rules and allowing Thich Nhat Hanh to return at all. Perhaps they just want to reassert their control and remind everyone who is really the boss.