Actually it says they gave the citizenship test to 1000 Americans to see how they would do. 29% didn't know who the VP was (so 71% did). 1000 isn't a massive sample, but I expect it's enough to say that the majority of American adults do know. Anyway, it isn't a very interesting discussion regardless.
I stand by my assertion that the majority of Americans have no clue who Biden is and I swear to you, I wish I didn't either. The whole political arena is absolutely revolting and I'm not surprised the majority of people are apathetic to it.