The problem is not only the +30 second lead. It's the time the 30 second lead was achieved in. After the second safety car, Vettel was just driving away from Rosberg with 2+ seconds a lap. That's an unbelievable difference, with Rosberg also racing for a top team (namely Mercedes, same team as Hamilton). There are also speculations Rosberg was on old tires and held the rest of the pack up, but still, 2+ seconds a lap is incredible.
It's also worth remembering that once Vettel builds up enough of a lead, he's likely to ease off a bit. If he kept pushing, his lead at the end of the race would likely be far bigger than 30 seconds.
