The T'Pau was a retired Vulcan ship that vanished from the Qualor II surplus depot and was used by the Romulans in 2368 as part of a disguised invasion force bound for Vulcan.
The T'Pau is central to the two-part Next Generation story Unification. Wreckage of a Ferengi cargo shuttle is found to contain a Vulcan deflector array, which is traced to the T'Pau, a ship sent years earlier to the Starfleet depot at Qualor II. When the Enterprise-D arrives, the quartermaster Klim Dokachin discovers that the ship is missing from its berth.
The trail leads to a smuggling operation and, in the end, to Romulus. Proconsul Neral and Commander Sela plan to send the T'Pau and two other stolen Vulcan ships to Vulcan, supposedly carrying a peace envoy but in fact holding two thousand Romulan troops.
When Spock, Picard and Data expose the scheme, a Romulan warbird destroys the three ships and their troops rather than let them be captured.