Duskblade will NOT be broken on VAYNE or any other ADC.
A friend of mine "Best Meguca NA" (I asked them permission to mention) and myself, "Ping"
Tested the theory of
and
On the PBE today.
They got fed early, and got Dusk blade as a second Item. They noted to me that it was actually a lack-luster building the item commenting.
"It takes a second" I asked about this and they went more in-depth saying that
#"The new dusk blade passive that deals bonus true damage, takes about a second of invisibility to actually proc."
In light of this information. Staying invisible just for that Proc, Vayne would actually lose a good portion of her DPS, than if she was to build normally.
The only ADC who may be able to pull it off could be Twitch, but I'm sure as well as other may agree that there are probably better items for the plague rat to build.
#I hope this new information puts a few of you at ease. :3
/ 
The build for the person you truly hate because you just wanna Q+AA and then walk away and watch them die. ;)