That's a bash thread if I ever saw one - no intention of being objective by the OP.
First of all, what you mention is not "poor". It's not on the level of Hamilton or Hulkenberg in junior series, but still got him the "rookie of the year" award and a second place. You do not seriously suggest that he was promoted that fast by being poor? Apart from that, Vettel is a driver who was improved each year even in F1. It's not sensible to believe that he was anywhere close to his peak in 2005 or 2006.
Did you also consider the age of a driver? There is a reason why Vettel has all those "youngest ever" records. Being a year younger or older is a lot in junior series.
People also mentioned that Vettel had many duties, whereas Di Resta could focus on F3. Here is a quote from Theissen:
Theissen, who has been a big supporter of Vettel for several years, goes as far as suggesting that the young German's failure to win the F3 title was directly related to his F1 responsibilities.
"Yes, I think so," he said. "He would never admit that, and certainly when he is in the car he doesn't feel that. But there is a certain amount of work that you can do and if you go beyond that then quality will suffer."
http://www.autosport...rt.php/id/55728
Keep in mind that test drivers back then usually had more work than today and you'll realise that is a full schedule for an 18 year old driver.
His accident was mentioned already - another reason why he wasn't always at a 100%.
Overloading with duties, young age, an accident, - more than enough reasons to miss the championship by a small margin.
Add to this that he dominated Formula BMW and led Formula Renault - and suddenly you have a very exciting junior career which can by no means be described as poor.