I've read a lot of the Snape isn't evil threads but haven't seen this variation yet.  

After another reread, I began to wonder if Snape made the vow w/out knowing the details of what LV had asked Malfoy to do.  He obviously infers that he knows...interrupting Narcissa when she is about to tell him Malfoy's task.  However, that may have been Snape's blustering attempt to prove to Bellatrix that he is closer to LV than she is.  Then, to put a stop to B's questioning of his loyalties, he agrees to make the vow w/out actually knowing what he's agreeing to.

Granted, Snape is a calculating person so agreeing to something this binding is something he would normally do.  However, he is also very arrogant and might have thought he could accomplish most anything...especially some menial task that LV had given to a child like Malfoy.  

There are other times in the book when Snape is question Malfoy...who seems to believe Snape doesn't really know what the task is.  It isn't until after Malfoy's first botched attempt at killing DD that Snape realizes what he's agreed to do, which prompts his heated discussion with DD.  

What do you think?

It's a good theory, it might be possible but I have a feeling Snape knew along what Malfoy's task was. But I'm a little undecided if LV actually told Snape or if Snape used legilimency on Narcissa. When Snape was making the vow,  Narcissa asks Snape if he will do Draco's task if he fails, Snape's hand flinches for a second. He knew what Malfoy had to do.

He even guided Malfoy somewhat;he knew about the necklace and about the mead. Malfoy started using occlumency against Snape because he didn't want his help. He didn't want Snape to get the glory, as he put it.

The UV, I  think he did it to prove to Bella that he is loyal to LV.  He didn't know actually what Cissy would make him vow too, just to protect Draco and look out for him. So when he agreed to the vow, he was agreeing as to what he said before that he would look out for him.  Once he was doing the vow and Cissy said the third thing then his hand moved and he was screwed.

Plus the fact that Snape actually took the vow proves he is not on LV side, since the vow was behind LV back.  I think Snape did the vow so Bella would shut up and to prove to her that he was on LV side, this proves he is not on his side b/c only a guilty man feels he has to prove he is innocent.

I agree with all of the points so far - Snape had no idea what Malfoy's task was. I also think that one of his main motivations is to protect Draco. There is a common thread throughout the books of Snape doing his best to protect students from harm - Harry on several occasions, Neville in Umbridge's office in book 5 etc.  Especially in The Flight of the Prince chapter, he is protecting Harry as well as Draco. Think of all the nasty spells Snape knows - hell, he invented some of them!, but all he does originally is block Harry's spells and prevent a DE using the Cruciatus Curse on him (since when has Voldemort been against one of his enemies being tortured a little?) He also continues to ensure that Draco is safe, even though the Unbreakable Vow would have been lifted when Dumbledore died. This chapter proves to me whose side Snape is really on, and it isn't Voldmort's. If Harry used his brain for a change he might even realise this.

how do you know he didnt know of the plan? when mentioned in DH he knew what was happening. because, like DD said he would be the natural sucessor if Draco died. so you dont know for sure if he knew or didnt know