Mark Vargo was a former Marine Corp Sergeant living in a town called River Grove, Oregon. He was also one of the people Sam Winchester saw in his vision.
He was the first person Sam recognized from his vision when he and Dean first arrived in River Grove. He was reluctant to answer their questions until Dean broke the ice by noticing the U.S. Marine Corps bulldog tattoo on his forearm, referring to him as Master Sergeant and mentioning his own father's unit in the Corps. He directed the Winchesters to the Tanner residence.
He was one of the people not infected by the virus that was spreading through the town. When Dean returned to the town after encountering a hostile roadblock while attempting to get help, Vargo threatened him at gunpoint believing him to be infected, but Dean managed to talk him out of it and they returned to the clinic together. After the infected townspeople left the town, he and Duane Tanner decided to clear out as well, and left together. Later, Duane slashed his throat in order to obtain the blood required to communicate with demonic forces.