How do you stake a vampire?

Once you've killed a vampire that needs to be staked, you have to find his coffin. The cursor will change to a sword when you move it over the coffin, and a two clicks will stake the vampire.

But the vampires in the steet how do you stake them?

You can't.

If there's not a coffin around, it's not necessary to stake the vampire. The gaseous form will just float away. There are only a handful of vampires that can be/need to be staked.