Here’s what I came up with. I guess it shows how close it could be:
Scenario 1
Webber: no wins, two second places, one third place;
Vettel: one win, two second places;
Result: 271:267- Webber wins.
Scenario 2
Webber: wins one, two second places;
Vettel: wins two, one second place;
Result: 281:274- Webber wins.
Scenario 3
Webber: wins one, one second place, one third place;
Vettel: wins two, one second place;
Result: 278:274- Webber wins.
Scenario 4
Webber: wins one, two third places;
Vettel: wins two, one second place;
Result: 275:274- Webber wins.
Scenario 5
Webber: no wins, one second place, two third places;
Vettel: one win, two second places;
Result: 268:267- Webber wins.
Scenario 6
Webber: no wins, three second places;
Vettel: two wins, one second place;
Result: even on points- Vettel wins by a first place finish.
Scenario 7
Webber: no wins, two second places, one third place;
Vettel: two wins, one second place;
Result: 271:274- Vettel wins
Scenario 8
Webber: no wins, three second places;
Vettel: three wins;
Result: 274:281- Vettel wins.
Scenario 9
Webber: three third places;
Vettel: three second places;
Result: 265:260- Webber wins.
Scenario 10
Webber: no wins, one second place, two third places;
Vettel: two wins, one second place;
Result: 268:274- Vettel wins.