the Wikipedia entry for Tango in the Night has this section:
When Nicks did go to the studio, she often felt unmotivated. "I can remember going up there and not being happy to even be there...I guess I didn’t go very often..." With vocal sessions taking place in Buckingham's master bedroom, Nicks would ask for some brandy, drink a few shots, and eventually run through "four or five songs" intoxicated. Because of this, Buckingham had to remove most of Nicks' vocals. As a result, Nicks is almost entirely absent on the majority of Buckingham and McVie's tracks. To make ends meet, Buckingham recorded some of the vocals himself using a Fairlight, an early sampling synthesizer. For example, on "When I See You Again", some of the vocals weren't even sung by Nicks. Instead, Buckingham assembled the vocal track by taking words and sentences that weren't hers, and then tampered with them until they somewhat resembled Nicks' voice. After the middle eight, the rest of the song is sung in Buckingham's normal register. 
I think whoever put this is got the song wrong. It seems doubtful to me that the vocals on Wassamatta Baby aren't Stevie but are his computer generated faux Stevie vocals. He may well have cobbled it together using bits and pieces from various takes, but clearly they are referring to a different song on the album, one of his or Christine's where as they say she is actually missing on most of them, and often when it sounds like her it's really tweaked versions of LB's own vocals.
But which song are they referring to that they mistakenly referred to as WISYA?