It is not just werewolves that have the weakness to silver; many vampires also have this weakness.
It appears to be a North American Folklore tradition and is not present in European Folklore
Modern vampires that have this weakness include:
Discworld Vampires -
On the Disc, all the world's vampire legends are true, even the
a catch all for anything ever said about vampire weaknesses **
- Vampires from The Necroscope series of books - throughout the books the alien vampires - Wamphyri - are shown to be weak against silver as their metamorphic flesh cannot abide it, it will paralyse them and cause them great pain and burning.
Then there was the Wamphyri fear of silver, which metal was a poison
to their system acting upon them as lead acts upon men
Necroscope 3 - The Source
But even the metamorphic flesh of a vampire has its weaknesses, such
as sunlight, silver, kneblasch, and the sharp and splintery point of a
Vampire world 2 - The Last Aerie (note kneblasch is garlic and the Szgany are banned from growing it and from owning silver on the Wamphyri homeworld)
- Vampires in Abraham Lincoln: Vampire Hunter - Silver as a weakness for vampires is mentioned all the way through this film. The culmination of the film
is the train carrying a load of silver to be melted down for bullets
at Gettysburg, which turns out to be a trap as the silver got there
via a different route. Im pretty sure Lincoln's axe was silver too and
he punched one of the vampires with his fathers(?) silver pocket watch
causing considerable pain (may have been the fight at the end)
(the first 3 are pre Matrix)
Reasons for this weakness range from silver being a precious and pure metal and therefore holy, to the first vampire being Judas and the weakness ties back to the 40 pieces of silver he was paid with.
Therefore a weakness to silver is not wholly a werewolf weakness and Cain having this weakness doesn't mean he cannot be a vampire. It does actually tie in with him being a North American interpretation of a vampire.