You don't have to be an ass to be a world champion.
Damon Hill was apparently a nice fellow, even with his champion status. Jackie Stewart was no snot, but he won three championships (and a knighthood). There are more.
Whether because of his age or his relatively sheltered racing superstar upbringing, Hamilton has said some things which were purely rude. When he is polite, it feels contrived. Vettel has the ability to sound polite and sincere. Reality or perception? Who knows, we don't live or work with these drivers. But the difference in perception has nothing to do with media coverage (Vettel got plenty at BMW, STR, and now RBR) or the status of their teams, but it's the way they present themselves to the public and to the sports media.
Without doubt, Vettel is not without flaws. He seems to be make judgemental errors when racing. Too hard-nosed, like Hamilton. But unlike Hamilton, Vettel is willing to apologise, even to his own detriment. That sort of redemptive attitude gives him a one-up on Hamilton in the "nice bloke" scorecard.