Well it was Italy and the Italian fans were unlikely to cheer anyone winning but either Ferrari or an Italian driver preferable both.
But as for the boo-ing of Vettel, as was said some of that might be down to the fact that Vettel hasn't done anything to help his popularity. Disobeying team orders when told to hold station with MW to name but one. Crashin into MW a couple of years back and blaming MW for it. Incidents like that don't earn your points. It was the same for MS, Hitting DH in 94(?) and JV(97?) either taking themout or trying to take them out in order to win the championship.