Translation:We protest for freedom of Vietnam.
It's not so much specific freedoms as freedom for specific places or people. "We protest for freedom of speech" or "We protest for freedom of religion" are perfectly acceptable constructions, but "We protest for freedom of African Americans" runs the danger of being misconstrued.
"Freedom" is an uncountable noun. But "Freedom of Vietnam' is a specific freedom. It therefore becomes a countable noun and requires an article. In this case it is correct to say " we protest for "the" freedom of Vietnam". Change it or don't change it. But if you don't, native speakers will continue to get upset.