Facing Jiren’s true colors is a feat in and of itself, and Vegeta and Goku both manage to do so beautifully.
Of all the warriors of the Ampitheater of Dominance, Vegeta is the only warrior to earn the genuine and earnest praise of Jiren, even before the Grey opens up to others following the climax. He embodies the apex of Jiren’s ideals, that one should strive for power all on his own, without throwing aside everything he stood for. Vegeta bested Jiren’s mightiest ally, Toppo, even after the latter wielded power of gods, through his own code of honor and dedication to others. In his last stand against Jiren, Vegeta fought KNOWING he could not win, but for a sense of duty ingrained into his very psyche. For this, he had earned the title of “Warrior” from Jiren, WITHOUT the aid of divine power. Vegeta did not win, he did not overcome the wall. But instead, the Prince of All won the respect of ALL. He proved that power wasn’t everything, that there was strength beyond strength. And that is why the existence of Jiren, the most powerful of walls, is so beautiful and inspiring. To give our heroes something to test their own ideals against, to struggle against and SURVIVE.
Goku, on the other hand, wielded the power of Ultra Instinct, at first failing to earn nothing but slight amusement from Jiren. Only after Goku became fully endowed, drunk in angelic power, did Jiren become so disgusted with the abandonment of one’s own strength did he envelop Goku in his onslaught. When their battle came to a close, Jiren refused to look Goku in the eye, because the SaiyaJin had won using power that was not his own. But in the very next moment, Jiren’s opinion of him changed. Goku’s body cracked and fell apart from the stress of Ultra Instinct. He had tried as hard as he could, truly given his all, before finally collapsing.
It was at this moment that Jiren understood. Ultra Instinct wasn’t a mere tool, it was a strength that only Goku wielded.