V'Ger is a very alien being & while Spock detected his immense intelligence at long range via telepathy over light years, it was more of a signal calling for it than a real conversation.
Think of it as the equivalent to a telepathic light house signal. It's very much a one way communication.
When Spock initiated his Mind Meld, he was inside V'Ger's inner chamber, basically right near the sensor that looked like a Plasma Orb, he was fairly close instead of being light years away at that point.
Even though Spock couldn't handle the volume of information being shunted to his mind by V'Ger over telepathy, he tried and was knocked unconcious due to data overload.
Then, somehow V'Ger sent Spock's unconcious body floating back to Kirk.
For Spock, he had to be up close & personal to try to initiate a two way telepathic communication while only being able to receive telepathic communique at long ranges.
When Sarek initiated a Mind-Meld across ~ 1,000 light years with Michael Burnham, there was a previous traditional Mind Meld with Michael when she was a child that left a piece of Sarek's Katra within Michael.
That's how Sarek was able to have a two way communication over such great distances, although it was very taxing on Sarek to perform such a feat. Even then Sarek was only able to converse with Michael for ~ 2 minutes before the connection ended.