The Belgian was set for a points finish before a late tumble into 11th as Dan Ticktum made his way past to secure the final point of the Saturday race.
It proved irrelevant as Vandoorne was burdened with 24s added to his race time for failing to use both attack mode activations and dropped to the bottom of the race classification.
Explaining the issue, the reigning champion recalled that he had lost all of the steering wheel functions in the car early on in the race, which precluded him from talking on the radio and selecting the attack mode activation.
"It was quite straightforward, really - after five laps, I lost the dash and all the functionalities on the steering stopped working," Vandoorne told Autosport.
"I simply couldn't activate it. I couldn't hear anything anymore, so I tried to take it doing the procedure, but I knew I was gonna get disqualified.
"I didn't know what energy I had. I couldn't see anything. So yeah, shit day."
As Vandoorne had no readouts on his dashboard, he was unable to work out how much energy he was saving - and, to his surprise, he found that he was 4% up on useable energy compared to the cars around him late on.
He suggested that this was proof that the DS Penske package had good efficiency but was alarmed at the "trend" of the car's performance falling away from practice to the race.
"There's been a fairly clear trend right now where, for some reason, we are fast in free practice and we can't really convert it in a good result in qualifying," he explained.
"OK, [Saturday] was maybe a little bit better for me, and I managed to get into the duels for the first time this year, but we're still too far away.
"There's definitely some stuff we need to figure out to improve that. The race is hard to judge; I mean, the Porsches look obviously super strong.
"But then I think it's probably a more level playing field after that. I think we're very good on energy, but we've not been able to use it so far, and today I had nothing that was working.
"Apparently I was still like 3-4% up [on energy] on anyone. Don't ask me how!"
