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The vacancy next to Di Resta is one of only three across the grid for the 2013 season, which begins with the Australian Grand Prix in Melbourne on March 17.
French driver Jules Bianchi, the team's third driver in 2012, was expected to be announced as Di Resta's teammate.
However, he was present at the launch of Ferrari's new F1 car on Friday as he is a member of the Italian team's academy.
Former driver Adrian Sutil and ex-Sauber man Kamui Kobayashi have also been linked with the role.
Force India team principal Robert Fearnley refused to be drawn on the identity of the new driver at the launch, telling reporters: "We do not want to confuse the two stories."
The VJM06 is the third of four cars unveiled so far for the 2013 season that have jettisoned the stepped nose design that featured widely in last year's world championship.
The Indian team finished seventh in last season's constructors' championship, but saw German driver Nico Hulkenburg leave to join rivals Sauber at the end of the year.
Despite the uncertainty over the team's line-up, Di Resta is confident that Force India will make a stronger start to the season than they did last year.
"I think the pressure is on all the departments to make sure we start better," said the 26-year-old Scottish driver.
"Last year we underperformed, certainly until the European season kicked in.
"We did get some good results before that, in the rain in Malaysia and in Bahrain, where we just got everything together and managed to achieve something remarkable that early in the season.
"We need to launch a car, understand it, and bring a race package to it that needs to make the car quicker than when it is rolled out on the first day."
Force India have described the VJM06 as a "brand new car from the ground up" and Di Resta hopes it will allow him to improve on his career-best fourth-place finish in Singapore in 2012.
"I'm feeling very good, I must say," he said. "I'm working very closely with my team of people. We're all trying to do the best job possible.
"Whether it's managing time and making sure you're in the best frame of mind or whether that's on-track performance and the finer details of the car set-up, ultimately it's all about performance and results.
On the subject of Di Resta's team-mate, team principal Vijay Mallya said the team were closing in on a new driver.
"We had a shortlist and have considered every driver who is available," Mallya said.
"We are very close to making our decision and I can assure you that we will arrive in Australia with one of the most exciting pairings on the grid."
Discussing the technical specifications of the new car, technical director Andrew Green said the VJM06 still bore similarities to the design of its predecessor, the VJM05, despite a new look.
"It's a brand new car from the ground up, everything is new," he said.