In the scene in the gazebo where Baby and her family are watching Johnny and Vivian Prestman dance, Neil comes up to ask Johnny where Penny is but he taps Vivian on the arm instead of Johnny.
That was just to get the attention of both of them. ( Johnny and the other dancer) She was tapped on the shoulder, so she quit dancing which caused Johnny to quit dancing. No slip up there!
He tapped her arm so she would know that he wanted to talk to Johnny. Plus, the woman Johnny is dancing with is not Vivian. The woman is actually the woman who wrote the movie, Eleanor Bergstein.