Hey. Doing vibrato on an open string is, as far as I know, impossible. What they could be doing is playing the same tone of a string in another. Per se, you could play D on the G string and do vibrato, and it'd be the same D as the D string.
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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.
"This young wine may have a lot of tannins now, but in 5 or 10 years it is going to be spectacular, despite the fact that right now it tastes like crude oil. You know this is how it is supposed to taste at this stage of development." ~ Itzhak Perlman