No he thought he could beat Perfect Cell. That's why he let him. And he only had the advantage against Gokou when he went Oozaru. But after that he's always saying how he is the strongest for most of the time until he admits Gokou is the strongest and always will be in the Buu saga. Also he thought he could take Fat Buu himself. Which is why he koed Gokou.
He didn't think Cell would power up that much so he took the risk. A stupid risk mind you but he didn't think Cell could become so much stronger. As for his first fight with Goku, he had the advantage before he became Oozaru, Goku even stated he was stronger but after Goku used his all, that's when Vegeta got weakened & used his Oozaru form.