The target – Trinh Xuan Thanh, 51, who was in Germany seeking asylum – was spirited back to Vietnam last month, where he faces corruption charges that carry the death penalty.
One of the alleged agents involved in snatching him from Berlin's Tiergarten park was arrested by Czech authorities soon after the kidnapping.
On Wednesday, he was extradited to Germany, where a judge a day later ordered him to be held in custody.
He was identified only as 46-year-old Long N.H., in keeping with German privacy rules in judicial cases.
He is accused of working for a foreign intelligence service and aiding in an abduction, which each carry sentences of up to 10 years' prison.
German prosecutors say the suspect rented a Volkswagen van in Prague and drove it to Berlin, where several armed men on July 23 dragged Thanh into the vehicle before he was “taken against his will to Vietnam, where he is in state custody”.
Thanh's lawyer has told a Berlin newspaper that he may have been taken in an ambulance to an eastern European country and flown out from there.
The unprecedented case angered Germany, which summoned the Vietnamese ambassador, kicked out the representative of the Vietnamese secret service, and decried the “scandalous violation” of its sovereignty.
Foreign Minister Sigmar Gabriel warned of further steps over the kidnapping, which he said evoked “thriller movies about the Cold War”.
The one-party state of Vietnam has waged an aggressive anti-corruption purge but analysts say it is often driven by infighting within the wealthy business-political elite as much as a true commitment to ending graft.
Thanh, the former head of PetroVietnam Construction Corporation, has been accused of mismanagement that caused losses worth $150 million (127 million euros), and vilified in state-controlled media for flaunting his wealth by driving a Lexus.
Also facing an embezzlement charge related to real estate deals which carries the death penalty, he had quietly slipped out of Vietnam in July last year.
Days after his abduction, a stone-faced Thanh reappeared on Vietnamese state television, which reported he had turned himself in.