The price paid for such knowledge wasn't one even Vexthra knew how deep it went.
Of course, the enchantress knew that, and that was why she didn't say much. If she did, she would have paid a price worth her life. Just warning her made her pay a huge price.
So she kept quiet, knowing that although the price was huge, she wouldn't die. Unlike her, who doesn't have a heart, Vexthra does, and that was why she knew she wouldn't die instantly.
Of course, the fix wasn't something she knew, but based on her calculations, she would live.
Just knowing about the Night Riders wouldn't kill her. She was only suppose to sacrifice 100,000 years of her life span and a small portion of her divinity.
But she was wrong, which became clear when Vexthra was done reading the pages. Of course, it could have ended there, but Vexthra wanted to tell Kent despite her warning.
