From Proto-Italic *welnō, from Proto-Indo-European *welh₃- (“to hit, strike”) (whence also vulnus, Czech válka, Old Norse valkyrja). The past participle was analogously changed from Proto-Italic *woltos to vulsus; a more logical outcome would be *vultus. See pellō for a similar analogy.
Compare typologically a semantic shift in the opposite direction Russian дра́ться (drátʹsja) <драть (dratʹ) (akin to выдира́ть (vydirátʹ)).