C.E. Evans wrote:
Starfleet didn't go searching for the Borg for the same reason the US government today doesn't go searching for Bigfoot - it was just a rumor. The Hansens were like the 24th century version of the people who bring cameras into the woods in the hope that they can find a picture of the Abominable Snowman to sell to tabloids, at least in Starfleet's opinion.
This would be pretty inconstant with Starfleets mission to seek out new life and to defend the Federation.
You quoted the wrong person (Elias Vaughn in a Mask
But it's not really inconsistent with Starfleet's mission at all. There really wasn't much information about the Borg for Starfleet to go on at the time, except second-hand accounts from the El-Aurians (who may not even have known where the Borg originated from or where they vanished to over seventy years earlier).