Vietnam is tightening controls on dissent and targeting journalists. When President Trump goes to Hanoi next week for a summit with his North Korean counterpart, he should raise with the host nation the fate of two Vietnamese journalists working for Radio Free Asia, the U.S. government-funded broadcaster I lead.

Popular blogger Truong Duy Nhat has been missing for more than three weeks since he fled his home in Da Nang, Vietnam, for neighboring Thailand to seek asylum. We fear Mr. Nhat was abducted and taken back to Vietnam...