Actually it's She Who Must Be Obeyed that's trying to remember a horror thriller from the late '80s/early '90s: something about a young girl, held as insane in an asylum, who discovers she can teleproject her consciousness out of her cell, and interact with people elsewhere. She's evil, and kills people. Then -- well, actually, that's all I've got. If anyone has a clue as to what this might be, She Who Must Be Obeyed offers eternal gratitude for, oh, say 20 minutes or so. No more!

PS: Forgot to add that SWMBO thinks the title included the word VISION(S) or VOICES.

This sounds like Door to December, by Koontz.  Actually it was originally published under his pen name - Leigh something I think.  The little girl was abducted by her father and used in sensory deprivation experiments.  She develops telekinetic abilities and begins killing her abductors.  HTH!

Koontz used Leigh Nichols as a pen name.