From: http://countrymusic.about.com/od/awards/a/blcmaprocess_2.htm

“Vocal Event of the Year: 
An event is defined as a collaboration of two or more people either or all of whom are known primarily as individual artists. They must have performed together, as a unit, on a musical recording released domestically within the eligibility period with each artist prominently featured and duly authorized to receive billing on the event. Award to each artist.”

Couldn't get much more individual than Grace, or more different than Kenny.