The Oath Rods don't bind you to what you say: they bind you to what you believe when you speak. Aes Sedai are well aware that when they swear to "never speak a word that is not true," they are still capable of applying misinformation to the billionth degree. If they had said something like, "I will never deceive anyone"--then they might be in trouble.

By saying, "I will serve until the hour of my death," there are a few things to consider:

1. Verin is, as far as we know, the first Black Ajah who didn't really want to be a member.
2. I'd say she's also one of the only Black Ajah ever to have both known that she was going to die in an hour and still have the freedom to act.
3. Verin is, as far as we know, the only one who, when she was taking the Oaths, realized and considered (and therefore allowed herself) the possibility ofnotserving the Dark One during the final hour of her life.

You can nitpick almost anything to pieces if you try hard enough.