Voyager 6 was a deep space probe launched from Earth by the National Aeronautics and Space Administration (NASA) in the latter half of the 20th century. The sixth probe in the Voyager series, it was designed to collect data and transmit it back to Earth. (Star Trek: The Motion Picture)
Voyager 6 was equipped with an antenna and a ground test computer, for use in communicating with Earth. It was designed to transmit a signal relaying its readiness to transmit the information gathered, and to do so upon receipt of a code signal instructing the computer to transmit the data. (Star Trek: The Motion Picture)
During its mission, Voyager 6 disappeared into what was once called a black hole (see V'ger), emerging on the other side of the galaxy, where it fell into the gravitational field of a planet populated by a race of living machines. The inhabitants found the probe to be one of their own kind, primitive, yet kindred. Discovering its simple, 20th century programming, which directed Voyager to collect all data possible and return that information to the creator, the machines interpreted it literally, and constructed a massive vessel around the probe in order to facilitate that directive. On its journey back to Earth, it amassed so much knowledge that it achieved consciousness itself, becoming a living thing. In the 2270s, the entity was detected by Starfleet en route to Earth, and was intercepted by the USS Enterprise, whose crew was able to discover the fate of the long-lost probe. (Star Trek: The Motion Picture)
According to Star Trek Chronology, Voyager 6 was launched in 1999. According to Decker's line in the movie, however, it was launched "more than 300 years ago". This suggests a launch date sometime before or in the early 1970s. In the real world, the actual launches of the first (and only) two Voyager probes took place in 1977.
The fictional Voyager 6 probe around which V'ger was built, was actually a full-scale mock-up of the real world Voyager 1 and 2 probes of NASA's Jet Propulsion Laboratories (JPL). JPL's director John Casani agreed to loan the model to the studio in October 1977, mere months after the actual Voyager probes were launched in August and September that year. Then Star Trek: Phase II (the immediate predecessor of The Motion Picture) Producer Robert Goodwin reported in a progress memo, dated 21 October 1977, "After your conversation with John Casani at Jet Propulsion Laboratories, JPL has agreed to loan us the mock-up of the Voyager, to be used as part of our set as the interior of the Alien Spaceship [note: indicating that the name V'ger had not yet been conceived at the time]. Joe Jennings and Matt Jefferies attended a briefing in JPL last night in the Voyager and Joe Jennings will return there next week with Bud Arbuckle to get measurements so that we can incorporate this large full-scale mock-up into our plans for the set." (Star Trek Phase II: The Lost Series, p. 52) The text commentary on the Star Trek: The Motion Picture Director's Edition DVD even stated that JPL was willing to even go a step further and loan the production an actual engineering duplicate of the Voyager spacecraft, but the studio declined, saying that the risk of the duplicate being damaged on the set was too high.
The William Shatner novel The Return, where Kirk and Picard join forces to lead an assault on the Borg homeworld and end the recent Borg/Romulan alliance once and for all presents the theory that a Borg transwarp conduit consumed the probe rather than a black hole, and that the planet seen by Spock was in fact, the Borg homeworld. The suggestion continues that the Borg assimilated the probe, yet the assimilation went "afoul" and changed Voyager 6 into a more sentient being.
Star Trek: Legacy presented the theory that the 20th century Borg civilization was a peaceful race. When V'ger encountered them, they studied its programming, repaired the probe, and sent it on its task. When V'ger returned to the Sol system it could not find its creator, but "a biological infestation." The probe returned to the Borg homeworld and joined with them, and its programming propagated throughout the Collective. Something of a civil war broke out. Massive amounts of knowledge, including the location of Earth, were lost in the resulting conflict, and the Borg of the 24th century were born.