Many a times in a story we see many bad guys turn towards the good side. Same thing happened with Vegeta the lead villain in Dragon Ball Z series. He is one of the Saiyans who survived when the planet Vegeta exploded. Unknowingly he worked for Freeza who had destroyed planet Vegeta, but he had no idea about it. He came to Earth when by mistake Goku(the lead hero) revealed the secret of the Dragon Balls to his then dying brother Raditz(also on the bad side !). Seeing Goku who always remained one step ahead of him in terms of strength, he felt jealous and in continued to train on Earth. Eventually he befriended the members of Earth Z Force and married Bulma(:O). They have a son named Trunks and a daughter named Bulla.
Even after his transformation his heart was not totally pure as was seen in the episodes where Babidi uses the blackness in his heart to transform him and use him against Goku. But still he fought (with arrogance and without teamwork)against many other villains like the Androids, Cell, Buu. He was also seen in the next series Dragon Ball GT where he fought alongside Goku against Chevron. Both Goku and Vegeta rose to Super Saiyan Level 4, and the series also involved the first appearance of Vegeto!! Overall he may not be the strongest one(compared to Goku) but still his attitude and style will always top the list. After all he is The Saiyan Prince !!