Paul Platten, 38, of Wincanton, Somerset, sent messages and naked pictures of himself after contacting a fake profile on social media.
He was tracked down and confronted by the vigilante group after he arranged to meet the girl in London.
He was sentenced at Taunton Crown Court earlier having admitted the charges.
Platten admitted three charges of attempting to incite a child in sexual activity and one charge of attempting to engage in sexual communication with a child.
Judge James Patrick said: "You sent to someone you thought was a 13-year-old girl a number of photographs of you naked.
"In my judgment these offences are so serious that they warrant an immediate custodial sentence."
Prosecuting, Caroline Bolt said members of a vigilante group set up a fake profile on social media of a 13-year-old girl called Sarah Rose.
Although he arranged to meet the girl in London, he later called it off. But the vigilantes tracked him down with the information he gave in his messages.
Representing Platten, Rebecca Bradberry said: "He has lost his home, he has lost his job.
"Given the confrontation that has taken place and that this has been plastered on social media he is not able to go back to Wincanton and his job."
He was ordered to register as a sex offender and was made the subject of a sexual harm prevention order - both of which will last for 10 years.
A paedophile has been jailed for 12 months after sending explicit messages to vigilantes posing as a 13-year-old girl.