I am living in Thailand, travel to Vietnam, and speak from first hand experience. Christians practice openly in Vietnam. Maybe people want to deny it because it would dry up funding for their NGO/charity of choice?
You might read here.
http://www.ishr.org/activities/religiousfreedom/christiansinvietnam.htm
"However, the International Society for Human Rights (ISHR) observes with great concern that Vietnam has discriminated, harassed and persecuted religious communities for years. ISHR records that in 2001 alone several hundred believers - among them all important religious leaders - were arrested or put under house arrest."
The ISHR is a non-profit NGO that bases its work on the UN Declaration of Human Rights.
Persecution by govt. is persecution, no matter the reason. Maybe you want to deny it because it would dry up business opportunity for you if anything was ever done about it.