My dear Sissa, I wholeheartedly agree with your infinite wisdom...(I knew there was a reason we were "bestest internet buddies"....lol) I have always felt Snape to be on the good side and your theory makes a great deal of sense to me. It would also make me to better understand why JKR has informed Alan Rickman about the character of Snape in full detail..and why he alone knows his character's fate.....

Once I thought about the word "vanquish"......it made lots of sense. I totally believe that Snape will be the key character in Book 7, and that he will have to somehow work with Harry to kill the Dark Lord....(Here again, I must add that I think Snape's love for Lily, plays a part in this scenario)