Did Draco completely fail in his attempt before Snape shoved him out of the way and did his thing?

My thought is that if Snape had waited for Draco to completely lose it he would have had to actually kill Dumbledore because of the vow he took. Draco was still standing in front of Dumbledore with his wand in his hand even though his hand was shaking a bit. And Dumbledore was still defenseless. I do not think that Draco had lost his chance to kill Dumbledore before Snape intervened. Snape was therefore not bound by the vow then to kill the Headmaster. I think the Avada Kedavra curse was masked.