Nyck de Vries won the Valencia E-Prix after half the field ran out of useable energy and were disqualified.
De Vries took first from race leader and polesitter Antonio Felix da Costa for his second Formula E win.
Polesitter Da Costa, taking it from Vandoorne, started in front with a 1.7 second lead before the first safety car was called as Porsche driver Andre Lotterer sent Sebastien Buemi spinning on Turn 9 and his rear wheels stuck in the gravel.
Highlights: Britain's Dennis wins first Formula E title in Valencia
Safety car number two arrived when BMW Andretti racer Maximilian Gunther lost his rear end as he sped into the gravel on Turn 2. The race then restarted before another stoppage as Mitch Evans lunged into the side of Dragon racer Sergio Sette Camara at the Turn 9 apex, forcing the Brazilian into the gravel.
Da Costa was four seconds in front with De Vries trailing in second before a late safety car was called for Lotterer taking out Edoardo Mortara.
It resulted in a two-lap sprint to the finish, but Da Costa hit his energy limit and started slowiny down to Turn 1, allowing De Vries - who started down in seventh due to a five-place grid penalty for a crash in Rome - to overtake.
Other then also hit their limit which saw Dragon Penske Autosport racer Nico Muller come from 22nd on the grid to take second.
Vandoorne, who began in last place, remarkably finished in third to secure the unlikeliest of double podium for Mercedes.
Robin Frijns came sixth as Da Costa dropped down to seventh. Alex Lynn and Sam Bird followed while Lucas di Grassi, Jake Dennis and the Jean-Eric Vergne were the only racers left to be classified. Everyone else was disqualified.
'I am so happy' - Britain's Dennis wins first Formula E title in Valencia
Dennis claims maiden Formula E pole in Valencia