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The vow is made in the context of Voldemort having tasked Draco will killing Dumbledore as punishment for Lucien's failure. In the chapter Spinner's End we find:

“Then I am right, he has chosen Draco in revenge!” choked Narcissa. “He does not mean him to succeed, he wants him to be killed trying!”

When Snape said nothing, Narcissa seemed to lose what little self-restraint she still possessed. Standing up, she staggered to Snape and seized the front of his robes. Her face close to his, her tears falling onto his chest, she gasped, “You could do it. You could do it instead of Draco, Severus. You would succeed, of course you would, and he would reward you beyond all of us —”

I can't find anywhere in the text where it states Voldemort will kill Draco if he fails, but that is certainly the implication.

So the unbreakable vow is not just to kill Dumbledore at some indeterminate point in the future. It is to save Draco's life by killing Dumbledore if Draco fails. That means Snape would be breaking the vow if he didn't kill Dumbledore immediately after Draco's attempt/opportunity.

Later:

As DavidS points out in a comment, the third vow is:

And, should it prove necessary … if it seems Draco will fail …” whispered Narcissa (Snape’s hand twitched within hers, but he did not draw away), “will you carry out the deed that the Dark Lord has ordered Draco to perform?"

(emphasis mine)

The vow is made in the context of Voldemort having tasked Draco will killing Dumbledore as punishment for Lucien's failure. In the chapter Spinner's End we find:

“Then I am right, he has chosen Draco in revenge!” choked Narcissa. “He does not mean him to succeed, he wants him to be killed trying!”

When Snape said nothing, Narcissa seemed to lose what little self-restraint she still possessed. Standing up, she staggered to Snape and seized the front of his robes. Her face close to his, her tears falling onto his chest, she gasped, “You could do it. You could do it instead of Draco, Severus. You would succeed, of course you would, and he would reward you beyond all of us —”

I can't find anywhere in the text where it states Voldemort will kill Draco if he fails, but that is certainly the implication.

So the unbreakable vow is not just to kill Dumbledore at some indeterminate point in the future. It is to save Draco's life by killing Dumbledore if Draco fails. That means Snape would be breaking the vow if he didn't kill Dumbledore immediately after Draco's attempt/opportunity.

The vow is made in the context of Voldemort having tasked Draco will killing Dumbledore as punishment for Lucien's failure. In the chapter Spinner's End we find:

“Then I am right, he has chosen Draco in revenge!” choked Narcissa. “He does not mean him to succeed, he wants him to be killed trying!”

When Snape said nothing, Narcissa seemed to lose what little self-restraint she still possessed. Standing up, she staggered to Snape and seized the front of his robes. Her face close to his, her tears falling onto his chest, she gasped, “You could do it. You could do it instead of Draco, Severus. You would succeed, of course you would, and he would reward you beyond all of us —”

I can't find anywhere in the text where it states Voldemort will kill Draco if he fails, but that is certainly the implication.

So the unbreakable vow is not just to kill Dumbledore at some indeterminate point in the future. It is to save Draco's life by killing Dumbledore if Draco fails. That means Snape would be breaking the vow if he didn't kill Dumbledore immediately after Draco's attempt/opportunity.

Later:

As DavidS points out in a comment, the third vow is:

And, should it prove necessary … if it seems Draco will fail …” whispered Narcissa (Snape’s hand twitched within hers, but he did not draw away), “will you carry out the deed that the Dark Lord has ordered Draco to perform?"

(emphasis mine)

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The vow is made in the context of Voldemort having tasked Draco will killing Dumbledore as punishment for Lucien's failure. In the chapter Spinner's End we find:

“Then I am right, he has chosen Draco in revenge!” choked Narcissa. “He does not mean him to succeed, he wants him to be killed trying!”

When Snape said nothing, Narcissa seemed to lose what little self-restraint she still possessed. Standing up, she staggered to Snape and seized the front of his robes. Her face close to his, her tears falling onto his chest, she gasped, “You could do it. You could do it instead of Draco, Severus. You would succeed, of course you would, and he would reward you beyond all of us —”

I can't find anywhere in the text where it states Voldemort will kill Draco if he fails, but that is certainly the implication.

So the unbreakable vow is not just to kill Dumbledore at some indeterminate point in the future. It is to save Draco's life by killing Dumbledore if Draco fails. That means Snape would be breaking the vow if he didn't kill Dumbledore immediately after Draco's attempt/opportunity.