I never bought the "Borg made Vejur" but it was a Roddenberry idea.
One day I will find the exact quote again but, IIRC, Roddenberry was being very lighthearted in an interview after the first Borg episode (TNG's "Q Who") and said something like, "Who knows, maybe the machine planet that repaired V'ger would have been of interest to the Borg?"
I don't think he meant that the machine planet itself was the Borg Homeworld. (That was Shatner's interpretation for one of his Return of Kirk novels.) After all, the V'ger Probe fails to acknowledge that the infestation of "carbon units" could be "true life forms". Most Borg drones are "carbon units" with enhancements.
OTOH, one might (must?) assume that Voyager traveled back in time as well as space. That gives it enough time to explore "the universe".
Yes, I think Christopher Bennett speculates this in "Ex Machina"?
Whew! I think I've hit everything. It's a great book. It distills the best parts of the movie and adds the kind of "definite take" that only Roddenberry could manage. And I think he does it well.
Me too. I still remember my first time reading it in December 1979. My first text exposure to Trek after "Mission to Horatius".