The Lions join Hertha BSC Berlin, who went up last week by way of their own late show against SV Sandhausen 1916, in securing a top-two berth, with third-placed 1. FC Kaiserslautern 13 points behind with only four games left to play.
Vrancic's strike takes Braunschweig back to the top flight for the first time since they were relegated in 1985 and completes a remarkable turnaround for the club, which was languishing in the depths of the 3. Liga just four years ago.
Torsten Lieberknecht's side certainly didn't have it all their own way in Bavaria, with chances few and far between in the opening period.
With the match maturing and the players beginning to tire, Sebastian Leitl's long-range strike which sailed narrowly wide seemed to spark Braunschweig into life. Suddenly top scorer Domi Kumbela was tearing at the hosts' defence at every opportunity, but the Congolese striker spurned two good opportunities, sending each wide of the right-hand post.
Vrancic the hero
At the other end Marjan Petkovic showed good reactions to keep out Ümit Korkmaz's awkward deflected low shot with his foot and when Kumbela was forced off with a hamstring injury shortly afterwards it looked as though the celebrations would be put on hold for another week.
But then up stepped Vrancic two minutes into stoppage time to curl over the wall and into the top corner and send the visiting fans wild and Braunschweig back into the Bundesliga they sensationally won in 1967. This season's second-tier title is still within reach, with Hertha one point ahead but with a game in hand, against FC St. Pauli on Sunday.
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