Harry didn't have a prophetic vision. It was just his imagination.
You seem to be confused about what sort of "vision" Harry had. Here's the passage in question.
"It's not easy, Apparition, and when it's not done properly it can lead to nasty complications. This pair I'm talking about went and splinched themselves."
Everyone around the table except Harry winced.
"Er - splinched?" said Harry.
"They left half themselves behind," said Mr Weasley, now spooning large amounts of treacle onto his porridge. "So, of course, they were stuck. Couldn't move either way. Had to wait for the Accidental Magic Reversal Squad to sort them out. Meant a fair old bit of paperwork, I can tell you, what with the Muggles who spotted the body parts they'd left behind..."
Harry had a sudden vision of a pair of legs and an eyeball lying abandoned on the pavement of Privet Drive.
(Goblet of Fire, Chapter 6, The Portkey).
As Harry Johnston points out, this wasn't intended to be described as a prophetic vision. If you recall, Harry never has any great aptitude at all for Divination or fortune-telling in the books. He wasn't predicting the future. He was letting his imagination run wild and trying to picture what the aftermath of a Splinching might look like.
The meaning of vision in this context refers to
a vivid mental image, especially a fanciful one of the future.
plural noun: visions
"he had visions of becoming the Elton John of his time"
an experience of seeing someone or something in a dream or trance, or as a supernatural apparition.
"the idea came to him in a vision"
Consequently, the 'vision' Harry sees never comes to pass in the later books. It's just a picture that momentarily comes into Harry's mind.
Splinching as a concept does, however, feature in the story. Ron gets splinched.
The moment his eyes fell upon Ron, all other concerns fled Harry's mind, for blood drenched the whole of Ron's left side and his face stood out, greyish white, against the leaf-strewn earth. The Polyjuice Potion was wearing off now: Ron was halfway between Cattermole and himself in appearance, his hair turning redder and redder as his face drained of the little colour it had left.
"What's happened to him?"
"Splinched," said Hermione, her fingers already busy at Ron's sleeve, where the blood was wettest and darkest.
Harry watched, horrified, as she tore open Ron's shirt. He had always thought of Splinching as something comical, but this...his insides crawled unpleasantly as Hermione laid bare Ron's upper arm, where a great chunk of flesh was missing, scooped cleanly away as though by a knife.
(Deathly Hallows, Chapter 14, The Thief).
As does Susan Bones.
Not until the fourth [attempt] did anything exciting happen. There was a horrible screech of pain and everybody looked around, terrified, to see Susan Bones of Hufflepuff wobbling in her hoop with her left leg still standing five feet away where she had started.
The Heads of Houses converged on her; there was a great bang and a puff of smoke, which cleared to reveal Susan sobbing, reunited with her leg but looking horrified.
(Half-Blood Prince, Chapter 18, Birthday Surprises).
It's evident from these descriptions that Splinching is a good deal more painful and dangerous than Harry imagines. His idea of Splinching isn't accurate; it's just a casual guess at what it might entail.