Nappa would ask Vegeta what the Shinigami's power level is, resulting in an answer of "It's over 9000!", and Vegeta crushing his scouter. Then, depending on which shinigami he faced, and assuming it was someone of a vice-captain level or higher, it'd be an interesting fight, considering Nappa, as a high level Saiyan, did have a great deal of speed, so he might be able to keep up with flash steps. His great strength would probably be if he was able to close to point-blank range with his opponent without losing a limb, however that would be unlikely. Ki blasts could be compared to cero blasts, and would still present a danger.