Vegeta, Prince of the Saiyans, made his debut early in the Dragon Ball Z anime adaptation of Akira Toriyama's Dragon Ball manga. Short in stature and undeniably evil, Vegeta made a big impact during the Saiyan story arc when he faced off against Goku, another Saiyan and the series' protagonist. Vegeta was forced to retreat from the battle after an embarrassing loss at the hands of Goku's son, Gohan, and his friend, Krillin. After that, Vegeta had to stand by and watch as Goku's powers grew to surpass his in nearly every way.
And although Vegeta's character has arguably gone through more personal development than any other main Dragon Ball character, his ongoing rivalry with Goku has yet to provide a clear answer as to which Saiyan is truly the most powerful. Even in Dragon Ball Super, the contest between Vegeta and Goku remains a central theme and has spread to a bitter ongoing debate among the series' fans about which hero should be the most powerful Saiyan warrior. Should it be the ambitious and uncompromising Vegeta or the noble and self-sacrificing Goku?
There has been some speculation as to why Toriyama seems to mostly favor his lead character, Goku, never allowing Vegeta to hold much of a lead over his counterpart. Toriyama has even stated he doesn't like Vegeta very much. Could that be why Vegeta never surpasses Goku? Keep reading for more theories about why Vegeta never passes Goku.
So this (insanely in-depth) theory actually comes from Redditor /u/thelamogio1 who states,
The reason why Vegeta will never beat Goku is because they are different types of fighters. Goku has been training all his life to be a duelist. He initially received training from master Roshi (a duelist and tournament champion) with the goal to win tournaments. Even his later training (Kami, King Kai etc) is focused on dealing with a single very powerful opponent, not a sizable group of enemies.
The only time he ever faced a real army was when he fought the Red Ribbon, who were just a bunch of assh*les with ak47s (and lots of cheap vehicles) not powerful enemies (they were many but come on Goku IS bulletproof, they might as well be unarmed!).Vegeta on the other hand has been training to be a soldier. He received training from King Vegeta, Nappa (both soldiers) and Freeza (a conqueror).
His early battles are against the armies of entire planets and he fought as part of a group like a soldier (or a special forces operative to be more precise). He often uses ki attacks like "Continuous Energy Bullet" which would obviously be devastating against a large group of weaker enemies and knows how to lead a group of soldiers (he often rallies the rest of the Z fighters, or knows how to "motivate" saibamen for example)
EDIT1: Plus Vegeta has shown that he knows a thing or two about guerrilla warfare as he has exhibited during the first days he was on Namek (attack freeza troopers only when he has the advantage, use tactics that would force them to split their forces etc) EDIT2: Also if you think about it Vegeta's current training regime focuses on him fighting multiple enemies/being attacked by all sides.
My theory is that Goku vs Vegeta is like pitting a MMA champion against a navy SEAL. The MMA champion will obviously be better in 1vs1 combat, but the special forces guy has the skills and training necessary to fight and win a war.
Vegeta has taken quite a few beatings during his battles, but no one takes a beating better than Goku, which is why healing is an ongoing theme in the Dragon Ball series. Every time a Saiyan is healed, their power increases dramatically. The Saiyans' power also increases after being killed and then resurrected from the dead via the use of the Dragon Balls. Both Goku and Vegeta have been resurrected twice. But how many times has each been healed after a battle?
Unfortunately, it is not clear just how many times Goku and Vegeta have been healed after sustaining injuries in battle and it is also unknown how many times Vegeta had to be healed of his injuries during his service to Frieza. However, throughout the series, Goku has definitely been pushed to the brink more times than Vegeta. So if being healed of physical damage increases the power level of Saiyans, it is likely that Goku has suffered far more battle scars than Vegeta and benefited from it. After all, Vegeta was originally more powerful than Goku and less likely to be injured to the degree that Goku has repeatedly been. In fact, prior to his battle with the Z-Fighters on Earth, it doesn't seem as though Vegeta was very familiar with the concept of defeat, let alone grave injury.
It just may be that Goku's willingness to take a beating has made him more powerful than Vegeta.
In Dragon Ball Z: Resurrection 'F', Vegeta and Goku apparently possess power of nearly equal skill and potency. However, Whis tells Vegeta that the reason why he continues to trail Goku in power is because he "thinks too much," he walks around with a chip on his shoulder, and his nerves are wound too tight. But one can surmise that this disadvantage is only a minor one. So, does Goku simply possess a natural talent that Vegeta does not?
The only other Saiyans who have ever reached Vegeta and Goku's level of power were the original legendary Super Saiyan (whose identity remains unknown) and the Super Saiyan Broly. However, Vegeta, for all his talent, has never managed to demonstrate the same degree of raw talent that Goku has. This despite having a more rigorous training regimen than Goku. It just may be that Goku is the Michael Jordan of Saiyan warriors and Vegeta just doesn't measure up. Of course, that would never be something that Vegeta himself could ever accept.
Originally, Goku was a very angry and violent child under the care of his adoptive grandfather, Gohan. But after waking up from a coma caused by a head injury that nearly killed him, his personality changed radically and he lost most of the heightened Saiyan aggression that he possessed before the accident. This is what likely instigated his morally pure and strong-willed personality. His jovial demeanor and empathy towards others, including his own enemies, has resulted in a good deal of trouble for the hero. However, it is also what is behind his willingness to sacrifice himself for the sake of others. His greatest strength comes from his need to protect others.
Vegeta, however, is a different story. While Vegeta is also no stranger to sacrifice and selflessness, this has never come easily to the Saiyan prince. As a result, going all-out for others with no thoughts about his own well-being is a rarity. Vegeta values his own life too much to be truly self-sacrificing. He is hesitant to cross that line which Goku is just not afraid to cross. But is he weaker because of it?