Originally Posted by
Shai-Hulud
Are we talking about serial killers or spree killers/mass murders/rampage killers?
The later sort are people like Sabertooth, who just go hog wild in the open. Or Joker, who'll kill just anyone.
Serial killers operate in secret. For example, Hannibal Lecter. For years the FBI tried to catch the 'Chesapeake Ripper,' without knowing it was Lecter. Serial killers are usually highly selective.
The OP mentions 'serial killers' and I don't think Vash has any experience with them, if we're going by the official terms as they use them in behavioral sciences.
I may be reading into the OP too much. If OP literally means serial killer then Vash is going to have a difficult time of things, as this is mostly a mental game. If OP means mass/spree killers, then no, Vash knows who they are, he catches up with them, outfights them, and wastes them.