We've been through that, too.
Snape did not promise to kill Dumbledore.
He promised to KEEP DRACO SAFE,
EVEN IF that meant killing Dumbledore.
The Vow itself is what will keep Draco safe from Voldemort.
The death of Dumbledore would have been irrelevant.

You need to read that section in Book 6, chapter One again.
I'll see if I can find the explanation around here somewhere.

==========

Nope. Can't find the thread in this club.
It was something about "Slimy Gits need love, too."
I'll have to send you to my unfinished website.
Maybe I'll finish it someday, but right now I don't have time.

http://www.geocities.com/bombleby/vow.html