Was there some message he was trying to convey by naming him Victory-Peace? Is Siegfried an attested alternate name for Sigurd that I've just never heard of? Was it the librettist's doing?
"Siegfried" actually dates back to the 13th century Nibelungenlied. That's still younger than the Norse sagas, but can't be attributed to Wagner.
/u/SerLaron has it right
Was it the librettist's doing?
Wagner was his own librettist, which is pretty unique!