Just found some interesting info about this topic.
In the Ernst Jorgensen book 'Elvis Presley: The Complete Recording Sessions' he claims it is Millie Kirkham who sings on Are You Lonesome Tonight?.
On page 285 it reads" Elvis had also hired his favourite soprano singer, Millie Kirkham, as the only musican from his Nashville band to join him in Las Vegas, and for her he came up with a high,word-less countervocal for 'Are You Lonesome Tonight?' her voice soaring high above Elvis's own like a tormented soul crying an unbearable loneliness".
So who is right? Tunzi or Jorgensen?