Vicente Iborra then capped off the scoring with a late fourth as West Brom slumped to another defeat, leaving them still eight points adrift of safety with now only eight games of the season remaining.
It all started so well for the hosts as they broke the deadlock inside the opening 10 minutes, with Oliver Burke's low cross into the box picking out Rondon, who turned the ball home from close range.
Leicester responded well to that early setback, but it was West Brom who again came close to scoring next when Grzegorz Krychowiak was denied by the crossbar after 15 minutes.
That was a moment which could have changed the game, but the Baggies had dropped a league-high 18 points from winning positions ahead of this match and Vardy soon set them on the path to increasing that unwanted tally.
Mahrez - who was the victim of an internet hoax earlier this week claiming that he had retired - created the goal with a ball over the top of the defence which Vardy latched on to before firing a fine finish into the bottom corner on the volley.
The goal continued Vardy's record of having scored on each of his four Premier League visits to The Hawthorns, and he had the ball in the back of the net again before the end of the first half, only for this one to be ruled out for offside.
Leicester continued to look the most likely to go on and win the game in the second half, though, and Demarai Gray threatened twice before the Foxes did eventually take the lead through another high-quality goal.
This time Mahrez was the scorer, scooping a classy finish over Foster after substitute Iheanacho had released him with a chipped through-ball of his own shortly after the hour mark.
Foster was keeping the Baggies in the game and made another good save to deny Vardy, but there was nothing the West Brom keeper could do about Leicester's third as Iheanacho nodded Ben Chilwell's cross home 14 minutes from time.
Pardew's side looked well and truly beaten at that point, and their sixth successive defeat - a seventh in all competitions - was compounded late on when Iborra again punished the hosts via the aerial route, powering a head from Marc Albrighton's cross past the keeper.
The result sees Leicester reach the 40-point mark which should guarantee their Premier League survival this season, but things look bleak for West Brom as they remain eight points from safety having failed to pick up a single point from their last six outings.
It is the first time they have lost six Premier League games on the bounce since 2002-03 - when they were relegated - and leaves them needing a response when they take on Bournemouth at the Vitality Stadium next weekend.
WEST BROM (4-3-3): Foster; Nyom, Dawson, Hegazi, Gibbs; Krychowiak (Field, 59'), Livermore, Brunt; Phillips (Rodriguez, 70'), Rondon, Burke (Robson-Kanu, 83')
LEICESTER (4-2-3-1): Schmeichel; Simpson, Morgan, Maguire, Chilwell; Iborra, Ndidi, Mahrez (Diabate, 88'), Gray (Albrighton, 67'), Okazaki (Iheanacho, 60'); Vardy