Then how could they have existed back 900 years in the past as the Vaudwaar mention in "Dragon's Teeth"?
I don't see how there is any reasonable connection between V'ger and the Borg.
It's canon that the Borg are capable of time travel. (ST: First Contact).
What if V'Ger and Decker (I will refer them both as the Entity form now on), traveled back in time, in a far away region of space, and had their offspring. Maybe it was innocent enough at first; a new lifeform having new beings.
Maybe, overtime it got warped. Perhaps The Borg traveled through time when they starting assimilating.
You can't deny that there is a similarity between V'Ger and The Borg. V'Ger saved entire planets, cultures, races, etc. into Data, whereas The Borg assimilated cultures races into one society. Both involve the collection of 'data' in one way or another.