If they make a request for aid, then no it's not.
Absolutely right. A lot of people don't understand this. The PD doesn't forbid any
intervention under any circumstances. When dealing with warp-capable cultures, it only precludes unwanted intervention. You can't take sides in a war between two alien powers or a civil war within an alien society, but you can offer to serve as a neutral mediator. You can't force another culture to change its ways, but if the legitimate government requests Federation aid, then you absolutely can provide that aid.
Sure, if the Federation had a cure and the Vidiian government had refused to accept it, then their hands would've been tied. But that doesn't seem likely, seeing as how the Vidiians were desperate to find a cure. We know that eventually the Think Tank provided a cure, and given their mercenary methods, it's one they surely would've withheld unless the Vidiians had agreed to pay their price. So since the cure was administered, that means the Vidiians must've accepted it, must've asked for it. And the Federation would've offered their cure free of charge, without demanding a steep price like the Think Tank presumably did. So there's no reason to think the Vidiians would've resisted the Federation's help, and thus there's no Prime Directive issue here.