Thanks to this post, this site has become the top hit on Google for such phrases as "I will have vengeance!" and "I will have vengeance, I will have salvation!" And lots of hits a day are coming through because of it.
I don't know if people are looking for ways to get vengeance and salvation, or if they're looking for the song the phrase came from in Sweeney Todd.
If it's the latter, that's easy: The song is "Epiphany" and the lyrics, wiht a fwe typoz, are available here. You can see it on YouTube in various productions: Len Cariou (the original Sweeney) and Angela Lansbury (the original soundtrack album is a masterpiece, like a bloody and brilliant radio show); George Hearn and Angela Lansbury; George Hearn and Patti LuPone; a little bit of Johnny Depp.
For actual vengeance and salvation, let me introduce you to my friend Nick Mamatas, who will be happy to help, I'm sure...