Wild Heart vocals album vs tour
 

Why are Stevie's vocals so different on the album from the tour?

On the album they are rich, almost over-powered, by her vibrato, and yet from what I can see from the US Festival performance, it is not as apparent.

Coke.
But also her Us festival performance wasn't very rapresentative both for her vocal problems and the audio problems of the night.
If you hear other bootlegs in which she's more sober and not strained the vocals aren't worse but a little more nasal and they're starting to be shrill. A preview of the Rock a little era.
But seriously, most of the time she had a stuffed nose and weary vocal-chords (like always).

Same with the Mirage album versus the tour.

It was at that time during the Wild Heart that the various substances she was abusing was just "starting" to get out of hand. So I think that might be a major factor why her vocals weren't up to snuff on that tour. The album, itself, was recorded in just twelve days...at least the musical skeleton and vocals, so her focus was there, lending to the rich, powerful vocals like you mentioned.

JMHO