i personally think LW is a better choice because ad carries like vayne have to be 100% careful with positioning and the armor reduction is only useful if it's stacked constantly on a focused target, but that's never the case in teamfights lol
i'd rather buy lw also because..
long sword + pick = lw
bf + dagger = bc
that bf.. that first BF should be built into a BT/inf edge, NOT bc
if you had leftover money after pd/bt or ie, then maybe bc is okay, but most people go lw/ga/qss because it works - ad carries have to move in & out of fights, not stay in it (which means you prolly cant have the armor reduction bonus the majority of the time)
just my 1.5 cents
edit: found this on league wiki
"Last Whisper becomes more effective at penetrating armor than The Brutalizer when an opponent has more than 37.5 armor.
It becomes more effective than Youmuu's Ghostblade at 50 armor. It becomes more effective than 3 stacks of the Black Cleaver's debuff at 112.5 armor."
if LW is more effective than 3 stacks of black cleaver at
112.5 armor, then LW > BC all day.. lol :P since nowadays, lots of ap champs will build zhonya after core items, junglers will naturally have this much armor if they run into lategame, and top champs that aren't ap always have 100+ armor @ endgame so i think LW is better
and LW is cheaper by like 600 gold :3