Perhaps I'm misremembering, but Vadic was not right when she said that one of their own had to steal the cure.
Bashir and O'Brien (two solid SF officers) stole the cure from Sloan's mind so they can cure Odo. I'm not entirely sure what their stance was on sharing the cure with the Founders as a whole, but they trusted Odo and for him to make the final call if I'm not mistaken.
So, there was a clear division in whether the Founders should be cured, but ultimately, they trusted Odo to reach the decision, which they had, and a cure was delivered to the Great Link.
Sisko, through the Federation Council and Starfleet Command, specifically orders Odo NOT to give the Founders the cure. And Odo outright states the decision is abetting genocide.
From DS9's "Dogs of War" (Season 7, Episode 24):
BASHIR: Section 31. They used you as a carrier, hoping you would pass on the disease to the Founders. I'm sure you were never meant to develop symptoms.
ODO: I don't care whether they meant to kill me or not. The reality is the Federation set out to destroy my people.
BASHIR: Section 31 aren't part of the Federation. They're a rogue organization that...
ODO: Don't split hairs with me, Doctor. They used me as an instrument to try to commit genocide. Now we may be at war with the Founders, but that's no excuse.
BASHIR: I completely agree.
ODO: And what does Starfleet intend to do about it?
SISKO: The Federation Council considered giving the Founders the cure, then they decided against it.
ODO: Then they're abetting genocide.
SISKO: I don't condone what Section 31 did, but the Founders started this war, not us. Giving them the cure would strengthen their hand. We can't do that. Not when there are still millions of men and women out there putting their lives on the line every day.
ODO: Well, I can see there's no point protesting. The decision's been made.
SISKO: Odo, I wish I didn't have to say this, but I need to know you're not going to take matters into your own hands.
ODO: You have my word.
SISKO: That's all I needed to hear.
ODO: Interesting, isn't it? The Federation claims to abhor Section 31's tactics, but when they need the dirty work done, they look the other way. It's a tidy little arrangement, wouldn't you say?
In regards to the Federation Council and SF being unwilling to share the cure with the Founders... that was understandable.
The Founders were willing to destroy the Federation by almost any means necessary and were NOT above using biogenic weapons on other species (remember the Quickening?).
So I found Vadic's claims a bit exaggerated and without any real basis. She blames the Federation for engaging in acts of self-preservation and defense after a rather unprovoked attack and enroachment into the Alpha Quadrant with major military forces... prior to this, the UFP and AQ were sending expeditions to explore the GQ and establish one or two colonies (which was understandable) presumably outside Dominion space.
I think the decision has parallels to the US's decision to nuke Japan, and how there were no really good choices, and the decision for the Federation to arguably engage in genocide can be rationalized as the least bad choice.
For some of the people that see the bombings as an atrocity, one of the alternative arguments put forward about the United States' use of nuclear weapons is that President Truman should have instead just ordered a strict blockade of the Japanese islands to force the Japanese to surrender. It's probably the closest option to the reasoning that says the Federation was justified in using devastating effects against an entire population as leverage to force capitulation.
However, the argument is
very morally dubious. It basically argues that it would have been "better" for the US to STARVE millions of people by withholding necessities to force surrender than to have killed thousands with Fat Man and Little Boy.