While a BMW test driver in the middle of 2007, the German marque released Vettel to race with Red Bull's junior team Toro Rosso.
Now just 22, the German stayed in the STR cockpit last year and then moved to main team Red Bull Racing this year, on a new two-year contract.
Moreover, if Red Bull is in contention for the 2010 championship, it is believed the energy drinks company will have the option of retaining Vettel for a further three years.
Theissen told the German daily Der Tagesspiegel that he doesn't regret letting Vettel slip through his fingers while opting instead for Robert Kubica and Nick Heidfeld.
He said Kubica and Heidfeld contributed to BMW's rate of progress between 2006 and 2008, and that the opportunity to reunite with Vettel may arise in the future.
"If everything goes well, Sebastian's career still has 15 years in it," said Theissen. "He can drive for us in the future."
Regarding BMW's 2009 driver lineup, Theissen said an announcement will be made "before September".