That's a direct quote from Boris:
He kinda contradicted himself.
He takes a moment to gather himself. "I would have liked him to have a proper end against foes of Kislev, but there is a better way now. Do it as quietly as possible, leaving no evidence. If the way you do it could only have been done with magic, that is good."
The first bolded sentence contradicts the second one, because if we
really leave no evidence, it will be impossible to deduce whether the killing was done by magic or not. So if he just keels over in some way indistinguishable from a mundane heart attack, Boris is going to be hard pressed to say "VAMPIRES DID IT" without people giving him side-eye. Primary objective fulfilled, secondary objective compromised.
I'm pretty sure what Boris
meant was:
1) "Leave no evidence
that a Grey Wizard did it, and especially none pointing to me" and
2) "Leave evidence
suggesting that a Lahmian is the most obvious likely suspect." and
3) "Leave evidence
suggesting, furthermore, that the Lahmian was at least trying to be subtle."
If Vladimir dies of a heart attack in public, we succeed at the first but not the second and third. If he dies of a heart attack in a locked room, we still succeed at the first but not the second and third- the Lahmian is, so to speak, being so subtle that the fictional Lahmian assassin
succeeds in deflecting suspicion.
If he dies of two small puncture wounds in his jugular vein in a locked room and the killer vanishes without visible means of egress, we succed at the first and the second conditions, but not the third.
If he dies of two small puncture wounds in his neck in a locked room, then the killer slashes at his neck in an attempt to conceal this fact and
then vanishes without visible means of egress, then we succeed in all three.