Spock's mind meld doesn't just give Spock insight into V'Ger - it's inability to understand "this simple feeling" - it gives V'Ger insight into the "carbon-based units infesting U.S.S. Enterprise".
It's been shown in the series during that the Vulcan mind meld, BOTH parties share emotions, thoughts, even memories. Thanks to Spock, V'Ger begins to understand its "lack" in not having emotions. Spock's feeling of disappointment that V'Ger's "pure logic" is not the answer goes into V'Ger and makes it understand that it NEEDS emotions to move beyond its programming of simply gathering data. After Spock's mind meld, it begins to understand WHY it is incomplete.
Proof? It tells Kirk and Co. that it NEEDS the information of why the creator has not responded. If it were simply a logic-based machine, it would not NEED. And the fact that the "Ilia unit" changes its tone at that point to a more pleading one is significant.
Which explains why the other movies don't reference it, why the uniforms change so drasticly starting with Wrath of Khan, and why in The Voyage Home no one says "What??!!?? Another giant probe is heading straight to Earth??? Awwww com'on!"
- Not enough lens flares...
- Also the explanation for the uniform change might be the same in-universe as it was in reality: the people wearing them hated them.
V'Ger is sent through a black hole to the far side of the galaxy, where a machine planet (the Borg in the 20th century?) fixes it up, upgrades it and sends it on its way. Later after returning to earth, the plot happens and in the end V'Ger ascends to a new level of existence (Q/Q continuum).
Simply put, Voyager VI fell into a black hole and was catapulted across the galaxy, ending up near the Borg homeworld, where the damaged probe was repaired and upgraded before being sent back towards Earth. This would indicate that the Borg began as cybernetic and only began assimilating carbon-based life-forms after creating Voyager. Though this would mean that the Borg became considerably less powerful over time. This theory does get some additional backing from a special feature on one of the DVD releases, an in-universe briefing (of dubious canonicity) which goes as far as to field it as a possible explanation.
- In Star Trek: First Contact, the Borg's leitmotif also uses the blaster beam from V'Ger's leitmotif. Both TMP and First Contact were scored by Jerry Goldsmith.
- There's also the fact that Ilia is assimilated by V'Ger, Spock says that "resistance would be futile" after melding with V'Ger's mind, and that Gene Roddenberry himself once suggested this as a possibility.
Kirk has his ship back, his Spock back, his McCoy back, and is in a very psychedelic bizarro world. What more could he want? And, it's not referenced anywhere else in the series.
- And this dream could have happened when Starfleet was picking their new uniform design, and the Interstellar Pajamas were Admiral Kirk's idea for new uniforms, but he was turned down and a more Navy-like style won.
- Or much later, inside the Nexus.
Decker says Voyager 6 fell into "what they used to call a black hole." But in later series black holes are still called black holes. So what Voyager 6 fell into wasn't a black hole but something people used to think was a black hole but turned out to be something else, probably some sort of unique wormhole capable of transporting the probe across the galaxy in one piece.
- Maybe it was a dig at a competing Disney film out at the time? A black hole? What's that?
- Terminology changes, along with the language in general. It is likely that the term "black hole" used elsewhere in the franchise is possibly some kind of Translation Convention so 20th century audiences could understand it. (Which makes one think- perhaps all the characters are speaking a slightly different version of English (or whatever language) to our English, which sounds like Antiquated Linguistics to them and if they time travel, they have to try and adapt as best possible. (Or use the Universal Translator which in accordance with Clarke's Third Law "magically" translates what they say into what we hear as perfect 20th-century English.)
The machine race that upgraded V'ger merely gave it the potential to fulfill its programming, by making it possible for V'ger to add capacity and capabilities as necessary. They had no way of knowing just how far V'ger had already come, or that it might eventually reach some threshold where it would decide it needed to return what it had found to its creator. This would have required the addition of some kind of analytical engine that could evaluate new situations and provide whatever upgrade was necessary to address that situation, whether it be additional storage capacity, a new type of sensor, or some other tool. It is this analytical engine that becomes the kernel of V'ger's sentience when it eventually upgrades itself to be more effective. The cycle of upgrades might explain why V'ger has so many layers inside itself.
- As for why V'Ger decided it needed to return, the Voyager probes were always designed to send information back to Earth, using radio. Clearly, after traveling across the galaxy, V'Ger was unable to reestablish contact with its creators, and these hypothetical analytical upgrades lead it to conclude that the only way to do so was to take the long road and travel back to Earth with the knowledge it collected on the way.