Bill Nighy is definately a good enough actor to portray the character of Kain, but i think i'd prefer to see the original back in the role. Perhaps a live action version of the series would be better suited for Bill Nighy, as like you said there is a similarity, but for this series i think CGI would be a better medium.
And please god no, not the guy who played Lestat in QOTD. Have you see him act in other things? You should, it's worth it just for the laugh... from what i recall he even got removed from the LOTR movies, not a good sign of a good actor. Good old Tom Cruise played that character better and that should tell you everything you need to know.