Second on track after Vandoorne kicked off proceedings in the battle for Saturday pole, Dennis took a slender advantage in the opening sector as the Belgian got very close to the Turn 2 barrier.
Dennis then opened the gap further, and although his slight slide in the final couple of corners helped Vandoorne find some time back, the Avalanche Andretti driver did enough to claim his second Formula E pole.
The British driver, who had said on Friday it would be "difficult" to repeat his 2021 win at the ExCeL before topping practice on Saturday morning, had earlier denied an all-Mercedes final with a 1m13.005s effort to dispatch Nyck de Vries.
The defending champion therfore takes third on the grid, having set a semi-final time faster than Sergio Sette Camara.
The Brazilian had made a surprise appearance in the semis, but could not overcome a mighty lap from Vandoorne, a 1m13.131s - over 0.3s faster than his Dragon Penske rival.
Vandoorne found a hatful of time over a tightly contested quarter-final bout with Andre Lotterer, who was ahead by 0.01s by the close of the first sector - but the Porsche driver failed to keep up that pace and began to fall behind, with the gap almost four tenths of a second at the line in Vandoorne's favour.
Sette Camara then bagged his first semi-finals place with victory over Dennis's team-mate Oliver Askew in their duel, finding nearly a quarter of a second over the American in the opening sector and cracking open the gap, logging a 1m13.330s to Askew's 1m13.688s.
De Vries made it two Mercedes in the semi-finals with a 0.4s advantage over New York City winner Nick Cassidy, as the Envision driver tagged the wall on his lap and lost time - thus, the Kiwi was denied a third consecutive chance to claim points for pole.
Dennis then set the fastest quarter-final time with a 1m13.225s to beat Maximilian Guenther into the final four. The Nissan driver was a late call-up into the final, as Venturi's Lucas di Grassi lost his laptimes in the group stages.
Askew's best time among the eliminated drivers puts him fifth on the grid for the race, ahead of Guenther, while Cassidy sits ahead of Lotterer on the fourth row.
Both Jaguars failed to progress from Group A, as Mitch Evans lost time with a lock-up in the first sector and was unable to make it through.
Guenther was just a shade away from making it into the duels, and although Vandoorne's final session-topping lap was enough to push the German out of the top four, di Grassi had his laptimes deleted for impeding Robin Frijns - causing him to angrily remonstrate with the live feed camera. The Brazilian will start from the back of the grid.
Both Venturis could not progress to the quarter-finals as Edoardo Mortara struggled for grip and confidence on his final lap, placing fifth in group.
This was ahead of both DS Techeetahs, as neither Antonio Felix da Costa nor Jean-Eric Vergne could progress into the final eight - dealing a big hit to the latter's title bid following a point-shy weekend in New York City.
Mortara thus assumes ninth on the grid, ahead of Frijns - fifth fastest in Group A - with the DS Techeetahs and Jaguars occupying the next two rows.
Da Costa starts ahead of Sam Bird in 11th and 12th, with Vergne ahead of Evans on the row behind.
