There is no widely recognized term or short phrase for what is going on here.
Your best bet is to just describe it as you see fit, trying to convey the meaning to whoever it is you are trying to communicate with. "We should use the 'Crazy' ending; you know, the accompaniment cuts out, but the vocals keep riffing". Given the difficulty in describing music with words, musicians cite existing music, or even coin new terms in order to communicate with one another, all of the time. Sometimes these exemplars become widely known, for example, this pattern at the end of a song is (can be) referred to as "the A-Train ending". Sometimes they'll just remain in a small community.