When he doesn't take advantage of his "near death experience" power ups.
Dragon Ball Z is absolutely full of Deus Ex Machinas: Sensu beans, fusion, the Hyperbolic Time Chamber and of course the freaking Dragon Balls themselves. One of the earliest of these was part-way through the Namek saga, where Vegeta revealed a secret ability of the Saiyans called "Zenkai". Basically, whenever a Saiyan is beat to the brink of death and recovers, he gets himself a powerup. Vegeta exploits this by having Krillin beat the shit out of him, and then Dende healing him. The boost Vegeta gets from this is so incredibly dramatic (if we're talking numbers, his "fighting strength" jumped from 30,000 to around 500,000) that it boggles the mind that Vegeta, Nappa and Radditz (or any Saiyan before them, really) wouldn't just be constantly kicking the shit out of each other and healing until they become Godly-powerful.