Ashton cited "serious concern" over a court's rejection this week of lawyer 53-year-old Cu Huy Ha Vu's appeal.
Hanoi's "failure to uphold the law and the fundamental right of all persons to hold opinions and freely and peacefully express them... is particularly disappointing in light of Vietnams good economic progress and its stated ambition to take a greater and more responsible place in the world," Ashton said in a statement.
She called on Vietnam to work with her "on these and other human rights and rule of law issues, so that Vietnams full potential can be realised."
Vu, the son of a revolutionary leader, was detained in November after twice trying to sue Prime Minister Nguyen Tan Dung over a controversial bauxite mining plan and a decree that prevented class-action petitions.
The European Union had already separately joined the United States, Canada and international rights groups in expressing concern about another Vietnamese dissident's re-incarceration.
Priest Nguyen Van Ly, 65, who has a brain tumour, went on a one-week hunger strike after authorities returned him to prison last month, his family said on Friday.