The Mercedes driver, who currently tops the championship standings, could only manage fifth place for the 60-lap race, more than half a second slower than Vettel's pole time.
While Hamiton was expecting to have a tough time of it in Singapore, he wasn't expecting such a gulf to the leading Ferrari.
“We know that we would come here and it would be difficult,” Hamilton confessed.
“I got everything I could out of the car, I gave it everything, everything and more.
“I don't know where Ferrari have picked up their pace,” the Englishman added.
“It's strange, from race to race it's a little bit different between us.”
With two Red Bulls and Kimi Raikkonen between Vettel and the two Mercedes, Hamilton isn't expecting his task to be made any easier in the race.
“This is a crap track for overtaking, it's usually just a long, long train.” he said.
“I think the start's an opportunity, strategy's an opportunity, safety cars, who knows.
“Tomorrow I'm just going to have to play the long game. It's a marathon, not a sprint, so I'll try and take that method into tomorrow.”