Only the Communist Party of Vietnam is legal to operate in Vietnam, other parties exist outside Vietnam or secretly operate inside Vietnam:

Communist Party of Vietnam
Đảng Cộng sản Việt Nam
Founded : 3 February 1930
Website : http://en.dangcongsan.vn/

People's Action Party of Vietnam
Đảng Nhân Dân Hành Động Việt Nam
Founded : January 18, 1991

Vietnam Reform Revolutionary Party
Việt-nam Canh-tân Cách-mạng Đảng
Founded : September 10, 1982

Bloc 8406
Khối 8406

Many members of these parties have been jailed in Vietnam under the charge of "attempt to overthrow the government". Political rights are very limited in Vietnam, especially right to change the government.