So recognize how hard it is for me to say this: V'Ger, the Whalesong Probe, and the Nexus make no sense.
V'Ger, in the original version of TMP, was generating an energy cloud "over eighty-two AUs in diameter", with energy readings almost off the scale. That size got scaled down somewhat in the Director's Edition to "merely" over two AUs in diameter. They tracked it moving from Klingon space into Federation space on "a precise heading for Earth". But the whole time it was, as you said, moving sublight. No indication it was warping space or tunnelling or any sort of go-faster tech.
The Whalesong Probe also was not moving FTL, for the Saratoga to intercept and scan it. When the ship had its power knocked out and started drifting, the Probe was still poking along -- it didn't immediately zip off at zuperluminal velocity.
Both of those were presumably making their way to Earth from wherever for over a century. The race that sent the Probe needed the lag time for the outgoing whalesong to stop reaching them before then sent it. And we don't know how long it took the "machine planet" that found V'Ger to build that ship around it and sent it back where it came from.
The "Solar Neighborhood" is roughly five hundred light-years radius from Sol. With the exception of Rigel, all the individual stars we can see are closer than that. FJ took a lot of that into consideration when he did his Technical Manual in the '70s. Greg Mandel likewise when he did his Star Charts. The fact that Rigel is so far away presents problems I won't get into here. But that means if V'Ger and the Whalesong Probe were traveling sublight, they could only come from a couple hundred light-years away, at most. And the fact that none of the races we know seems to have ever stumbled onto those planets is a stretch. But it could happen. They're just problematic as hell.
The Nexus is far, far worse, though. It circles the galaxy every 39.1 years. Which means, with the circumference out around where we are being about 314,000 light-years, the Nexus is booking it at about 8,035 light-years per year -- or warp 9.99, TNG scale. While it was certainly energetic, it also wasn't moving faster than light any of the times that we saw it. I highly doubt it would slow down when people were about and then piff off again at ludicrous speeds.