One of the most memorable scenes of Dragon Ball Z belongs not to its main protagonist Goku but his arch-rival Vegeta. In the scene from the "Majin Buu" arc, Vegeta sacrifices himself to save his friends and family. It's poignant for two reasons: it's the first time in the series that viewers witness Vegeta fighting for someone other than himself, and it's also an act that is inspired by Vegeta witnessing Goku earlier in the "Cell" arc.
The effect that Goku's sacrifice has on Vegeta, as well as the changes in his life, signal a profound change in the character. This is reinforced by Vegeta's recognition of his own crimes during this arc. With these together, Vegeta's heroic sacrifice would seem to indicate the culmination of his character arc -- an inspired, selfless act that atones for his earlier crimes and illustrates his transition from a ruthless and selfish fighter to a father, friend, and "true warrior." However, this is not actually the case.
Past the simple fact that Vegeta returns to take even more prominent roles in subsequent Dragon Ball series, there is a moment from another story that describes a more lasting and significant shift. While Vegeta's sacrifice is a sea change in his character, his acknowledgment of Goku's strength in the subsequent "Kid Buu" arc is the true mark of his dramatic maturation over the course of the series.
What makes Vegeta such a fascinating and relatable character is that his greatest source of strength -- his pride -- is also the source of his greatest weakness. With this admission, Vegeta finally begins to wear away at this internal handicap and begin the proper restoration of his wounded fighting spirit. Vegeta had previously made countless excuses to minimize Goku's obvious strength -- attributing it to some technique, some quirk of Goku's bloodline, his devotion to protecting his family and friends -- anything other than the simple truth. What's different at this moment is that for the first time, Vegeta willingly admits that Goku is the stronger of the two -- and most importantly, he offers no ego-saving excuses to mitigate this fact.
Contrary to the pattern of Dragon Ball, which emphasizes external changes, Vegeta's shift is internal. Yet what's most significant about this scene is what follows this admission: as Vegeta's characteristic self-loathing and self-deception fade, his determination suddenly returns as the clarity brings the ultimate goal into focus. While this seems paradoxical at first, it speaks to the character of the internal change.
Before, in the "Majin Buu," "Cell" and even "Frieza" sagas, the lengths to which Vegeta went to misrepresent the source of Goku's strength actually worked against his ability to match Goku's power. This patchwork of excuses always led to dead-ends that never acknowledged Goku's innate fighting spirit, and in turn, never directed Vegeta toward healing and cultivating his own. However, with the clarity brought by this admission, Vegeta is finally able to accept the truth his wounded ego had kept from him and at last begin the hard of becoming stronger, which continues in Super.
"Whatever Doesn't Kill You Makes You Stronger"
While it's true that Akira Toriyama tentatively slated Dragon Ball Z to end with the "Buu" saga, the subsequent series' have continued to grow Vegeta's character. In this extended arc, the clarity brought by his admission plays a foundational role. In Super, Vegeta's rivalry with Goku changes; the effect of his admission is not fatal to Vegeta's self, but rather fosters its development. No longer resenting nor emulating Goku, Vegeta's newfound clarity and self-acceptance permit him to find his own path -- a change illustrated by "Ultra Ego." More than simply another step on the path, Vegeta's Ultra-Ego truly rivals Goku's power and accomplishes what his admission at the end of the "Buu" saga begins: the reconciliation of the self.