Vagh was the Klingon military governor of Krios Prime in 2367 during the Kriosians rebellion against Klingon rule. Vagh believed Starfleet was aiding the rebels by sending them weapons. During the visit of the USS Enterprise-D to Krios, Vagh ordered the ship surrounded by Klingon warships.
Actually, they were aided by the Romulans. That same year, Romulan Commander Sela ordered the abduction of Lieutenant Commander Geordi La Forge and used a brainwashing technique to instruct him to murder Vagh, and thus draw the Federation into conflict with the Klingons.
Kell, Klingon ambassador, was a Romulan collaborator who wanted to dissolve the Klingon-Federation alliance. He lured Vagh onto the Enterprise-D so that La Forge could kill him. He asked Vagh to witness the investigation of the transported weapons to convince him of the Federation's good faith. This would allow La Forge to kill Vagh. The plan was thwarted, and Vagh arrested Kell, after evidence supported that fact that this was a Romulan plot. (TNG: "The Mind's Eye")