You're right about the Vow lasting the life of both wizards. I don't think I worded my response well, so let me try again:
The Vow that Snape made was to "...watch over my son Draco as he attempts to fulful the Dark Lord's wishes... and will you, to the best of your ability, protect him from harm... and, should it prove necesssary... it if seems Draco will fail... will you carry out the deed that the Dark Lord has ordered Draco to perform?"
The vow wasn't to protect Draco indefinitely, it was to protect him while he attempted to take Dumbledore's life. And Draco failed, so Snape did it himself. At that point, the vow was complete. If anything, Narcissa blew it by constraining the vow like that. I don't think Snape would have agreed to protecting Draco forever - it's a selfless act, which doesn't ring true for Snape in my eyes.