Direct Hit.Yes that's true.But think.
Moments from being hit,Cell found that Vegeta's Blast could killed him.So,he was just about to jump away from there,when the blast touched his arm.
Gohan was able to finish off Cell when he was Super Saiyan 2,and with some help from his father.
So,how would Vegeta,from his Super Saiyan 1 level 3,finish off Cell in one blow?