The impostor Captain Tuvok was a Changeling who, sometime before Frontier Day in 2401, replaced the Vulcan Starfleet officer of the same name, during their infiltration of Starfleet.
While seeking allies in uncovering the very Starfleet conspiracy this Changeling was involved in, Seven of Nine contacted Tuvok, believing it to be her friend, hoping he could put her in contact with Admiral Kathryn Janeway. During their conversation, Seven challenged his identity through both their dialogue and through voice analysis. Though initially passing her tests, he revealed himself after she lied during her establishing of the meeting point, suggesting Aklion VII – a planet any legitimate Vulcan would avoid due to anti-Kolinahr demonstrations being considered blasphemy– as a place to meet.
Once the Changeling was revealed, he taunted Admiral Jean-Luc Picard by impersonating Captain William T. Riker and telling him "I'm as good as dead." (PIC: "Dominion")
Following the destruction of the Borg, Admiral Beverly Crusher developed a way to detect the Changeling infiltrators who were all rounded up and arrested. Starfleet discovered that the Changelings, in a constant need for information, kept most of their targets alive, including the real Tuvok, who were all eventually rescued. (PIC: "The Last Generation")
This Changeling was portrayed by Tim Russ in Tuvok's form, and Jonathan Frakes in Riker's form.