[Quiet Man Cometh]

The VK test isn't to see if Replicants have emphathy. It's already known/assumed that they don't (this is much more obvious in the book with the scene with the spider) and the VK test finds them by testing that empathy. Rachel is a replicant but does not know initially, so her reactions to the questions appear to be genuine, and she almost passes but Deckard trips her up with a surprise question and she doesn't react quite fast enough to prove legitimate empathy.

It's worth reading the book because they approach to empathy is so different. Roy Batty's character is dramatically different, as is Deckard. Deckard is married, and there is a whole religious movement that works from empathy and emotion.

EDIT: there is also an electric sheep. ;)