Imagine winning one million dollars. Some people would spend, spend, spend and end up broke while others would invest and become wealthier. Vegeta is the former whereas Goku is the latter. When Goku passes a limit, he aspires to surpass it so he’s ready for the next strong guy. When Vegeta surpasses a limit, he aspires to gloat and showboat until someone bigger and badder puts him in his place THEN he trains to become better. He gets lazy. That’s why he didn’t really start training HARD until he met Goku. Before Goku Vegeta was used to his royalty and natural strength and just assumed because he had royal blood he was the best. Sure he went on missions and trained but he never had to overcome obstacles or faced foes who forced him to become stronger through vigorous training and not just rely on the zenkai boosts or ape form.