Kate said she had fired Vanessa and that it was her last day. Dennis tells Horatio that the other nanny, Mia, would now be joining them on their trip since Vanessa was gone. Delko then questions Mia about her being able to go to on the trip now that Vanessa was dead. If Vanessa had been fired, she wouldn't have been going on the trip anyway.
The safe in the rich family's home is a cheap 80-dollar steel box which never would be found in a upscale home - especially not mounted inside a wall.