In 2367, the Kriosians started a rebellion against Klingon rule. Vagh's forces had found hundreds of what appeared to be Starfleet weapons as well as medical supplies. He informed the Klingon High Council and they sent Ambassador Kell on the USS Enterprise-D to investigate the claim. Vagh had made up his mind and wanted to crush the rebels with force, but deferred to Kell's investigation for the moment.
Lieutenant Commander Geordi La Forge showed that the weapons were actually provided by the Romulans. Vagh wanted his people to check that conclusion, even though Kell was satisfied. In actual fact, not only were the Kriosians aided by the Romulans, but Commander Sela ordered the abduction of La Forge and used a brainwashing technique to instruct him to beam more weapons covertly from the Enterprise. Vagh found the weapons and ordered the ship surrounded by Klingon warships.
Kell was also a Romulan collaborator who used La Forge's condition to get him to murder Vagh. This would draw the Federation into conflict with the Klingons and hopefully dissolve the Klingon-Federation alliance. He lured Vagh onto the Enterprise-D so that La Forge could kill him during the investigation of the transported weapons. The plan was thwarted, and Vagh arrested Kell, after evidence supported that fact that this was a Romulan plot. (TNG: "The Mind's Eye")