Snape and Voldemort are different characters - Voldy was always evil - always hated his muggle father for leaving him. Voldy always wanted to hurt other children.
Snape was a good boy but just fell in with the wrong crowd.
I know we don't know that until DH but still i think we don't have to see the Vow. We can learn about it by other means. I have a feeling it won't be cut now - it is a relatively important chapter.
can i ask - since the first film came out do you imagine the actors in the role and what they would be like in the film?
I don't but then again not many of the actors i think capture their character completely but then again i suppose it is hard as they didn't know how their character was going to develop.