Well,
there's something you don't see every day -- a VampLex-Superman fight,
with Kryptonite, in which Clark uses his head and his powers to beat the scumbag without the usual oh-I'm-helpless-because-he's-magical/has-Green-K stuff -- or, indeed, with Clark
or Lois ending up at Luthor's mercy. Very refreshing, and doubly so to see Superman being properly super. The lead suit was a nice touch (and a tip of the hat to STAS?).
So now Lois and Clark are faced with a true dilemma: Superman doesn't kill, but how does that relate to the undead? One answer, that I understand to be Buffyverse canon, is that Luthor (or the clone, or whatever it is) is actually dead and has been possessed by a parasitic demon, in which case Clark shouldn't have a problem once it's explained to him. All dusting the vamp will do is put the body to rest (something Clark can verify by using his IR vision to check VampLex's body temperature); there's no soul there, since the original occupant has been pushed out of the body, and the demon doesn't have one. YMMV, though.
I shall be interested to see how you wrap this up, but kudos for having a sensible confrontation between
Superman and ol' grenade fodder.
Phil, Official FoLC Luthor Barbeque Chef