I've recently scouted out Mariposa Military base for the Brotherhood, and convinced the Elders to send the Brotherhood in to attack. But there is still no dialogue option concerning the sterility of the Super Mutants when speaking to Vree. Is it only unlocked after I have completed "Destroy the Source of the Super Mutants"?

I think the "sterility of the Super Mutants" is on Vree's autopsy report. To get the disk it requires 7 Intelligence & Speech (can't remember how much, though. 75+ or something). Mentats the freshmaker can help sometimes...

Also, Vree won't talk to you if she has a disposition toward you karma/reputation-wise.