Though they played in the Champions League after their title triumph, they have twice fallen away from the top four in successive seasons having looked well set for much of the campaign.
But they left it late in London to send Manchester City home disappointed as Nigerian frontman Iheanacho converted the winner against his old club 11 minutes after coming on.
Up until that point it had been a rather even affair in the curtain-raiser to the new top-flight campaign which gets under way on Friday when Arsenal host newly promoted Brentford.
Injury problems in defence for Leicester meant free signing Ryan Bertrand was given a start, while fellow summer recruits Patson Daka and Boubakary Soumare both came off the bench in the second period.
But it was old boy Vardy who looked most likely to break the deadlock, though he showed a bit of rustiness with his finishing, smacking a bounce shot straight at Steffens from 12 yards out before the American kept out a shot with his trailing right foot while diving to the left.