You can kind of "cheat" a vibrato on open strings by doing the move with your fingertip right about where the vibrating length of the string meets the nut. 

But it is technically not a true vibrato, since vibrato is supposed to go both below and above the target pitch, so it averages at the pitch intended for the note.  Since the pitch can't go lower than the open string at the nut, the "cheat" only swings between target pitch and a little above.

Close enough for some occasions though, I think.