Vegeta had a rough time during his battle with Frieza in Dragon Ball Z. He tried multiple times to scare Frieza and his men by talking about becoming the legendary Super Saiyan. However, despite his claims, he never achieved the form while on Namek. In the end, the Saiyan Prince was killed by Frieza and forced to leave everything up to Goku. The lower-class warrior realized the legend by being pure of heart and awakened by fury, or at least that's the claim. It was revealed in the Android Saga that Vegeta always had the same potential to become a Super Saiyan, but he couldn't do it in time for his fight with Frieza.
Source：https://www.cbr.com/dragon-ball-z-super-saiyan-vegeta-vs-frieza-namek/
