I reckon Vettel is racing against tougher opponents than when Schumacher was in his prime.
I think you're spot on there.
When Schumacher was racing for his previous teams, his teammates were always told to move over to let him through to win, i.e. Irvine at the A1-ring.
Vettel teammate is "not bad for a number two", and does challenge him for the races when his car lasts long enough.

Not only that we have other drivers like Alonso, Hamilton, and Button to contend with in a race.
Actually one thing I've noticed, both those drivers have issues with getting past other drivers if they don't get the lead at the first corner.

That said, Vettel did spend time at Toro Rosso, whereas Schumacher did very few races in lesser teams before he went to Bennetton and then Ferrari.