Ferrari said Enrico Cardile would head up a new Performance Development department.
Other roles stay the same, with Enrico Gualtieri overseeing the power unit, Laurent Mekies remaining as Sporting Director in charge of trackside activities and Simone Resta head of chassis engineering.
"We are making changes to the technical side of the organisation so as to speed up the design and development on the car performance front," said principal Mattia Binotto.
"A change of direction was needed to define clear lines of responsibility and working processes, while reaffirming the company's faith in its technical talent pool.
"The department run by Enrico Cardile will be able to count on the experience of Rory Byrne and established engineers such as David Sanchez. It will be the cornerstone of the car’s development."
South African designer Byrne, 76, was a key figure during Ferrari's golden era in the early years of the century with seven-times champion Michael Schumacher in a team led by Jean Todt with Ross Brawn as technical director.
Ferrari have not won a driver's title since Kimi Raikkonen's success in 2007 while their last constructors' crown was in 2008.
Vettel, a four-times world champion with Red Bull, is leaving at the end of the year with Monegasque Leclerc the face of the future.
Binotto said Ferrari had to "make a decisive change, raising the bar in terms of the responsibilities of the department heads."
"We have said it several times, but it's worth repeating: we have started to lay the foundations of a process which should lead to a new and enduring winning cycle.
"It will take some time and we will suffer setbacks like the one we are experiencing right now in terms of results and performance."