Sorry,I forgot to mention the most useful thing : to practice vibrato slowly.
Without mentioning that,my previous suggestion is useless
As far as watching other people demoing arranger keyboards,it's useful,no doubt,but I suggest you listen to some mind-blowing guitar piece where the player has great vibrato skills...there are so many pieces and players,but one piece in particular that had great impact on me as regarding vibrato,is John Norum's 'Shimmering Highs'.
The slow melody at the beginning was an eye-opener for me as regard vibrato,not to mention an ear-opener
Once you hear that kind of vibrato,it might spark more ideas as how to perform it on keyboard.
I still have not tried to emulate expressive guitar vibrato on keyboard,'cause I am busy with learning other things about harmony etc, but I sure will try it sooner or later