In the past, children with HIV in Vietnam have been adopted by American parents. US / VN adoptions were placed on hold in 2008 and have re-opened as of September 2014. At the time of this writing, the 2 US adoption agencies approved for processing Vietnam adoptions are Dillon International and Holt International.
According to Dillon Int’l: they have placed children with HIV, children with HIV are waiting for families, both heterosexual married couples and single applicants between 25-55 can apply (this may be an agency rule not a country rule). Only one trip about 2-3 weeks in length.