Vettel turned out for STR in the second half of the 2007 season before spending 2008 with the team.
The German recorded a fourth place finish in only his sixth race which was the team's highest finish after nearly two seasons in the sport.
After a string of good finishes in 2008, the 24-year-old then claimed Toro Rosso's maiden victory at the Italian GP.
Tost reveals that Vettel differed from other drivers who have turned out for the team.
"Sebastian came to us with quite a bit of F1 experience, so he was not the complete newcomer," Tost told the Official Formula One website.
"He'd been a test driver with BMW-Sauber and had already done a race where he'd got his first point. Then he came to us with a rather unusual level of experience for a Toro Rosso driver, and that showed in his first full season when he won the Italian Grand Prix."
The Austrian believes that like many great drivers, there will be no carbon copy of Vettel and that key to the German's success - and that of any young driver - is a process of self discovery
"There will never be another Sebastian Vettel, just as there will never be another Michael Schumacher or Ayrton Senna," said 55-year-old.
"What I think is crucial in the development of a driver from rookie to serious podium contender is that the driver discovers himself. There is no such thing as the unerring talent scout.
"Sebastian Vettel has become World Champion because he has discovered himself - his own capacity and ability. The outside world can only provide the necessary environment."
With Vettel currently running away with the Championship in a dominant RB7, many have questioned whether it is Vettel's car rather than the man himself who is the main cause of his current success.
For his part, Tost believes that the driver is key to overall success.
"When you look around, the really successful drivers stay with their teams for quite some time because the process of growing together - driver, engineers, car, tyres - all takes time and once you've grown into one single functional unit you want to keep that as long as possible," he said.
"When it boils down to the sheer facts, for me the driver is the key to success."