The Phoenix Reborn wrote: He knew when he agreed to that Vow that he was agreeing to kill Dumbledore if Draco was failing. He knew it all along. And he still agreed.
Well, he knew DD would let him kill himself. I mean, DD knows that death isn't the worst thing in the world. He agreed to the vow because he knew DD would agree.
And as a little side note, how did Snape convince LV that he really was on his side. If he could've fooled LV, I think he's not bad at talking his way out of trouble.