They needed an early goal and it was sparked for them by Mickey Bell, the left-back who joined Port Vale in the summer after nine years with City.
His challenge caught Scott Murray on the thigh and the free-kick went out for a corner. As Luke Wilkshire swung the flag-kick into the goal mouth Steve Brooker's header was parried by the keeper but he followed up to score from the rebound.
A terrible mix-up in the visitor's defence then saw ex-Sunderland striker Marcus Stewart roll Wikshire's pass into an open net and the match was barely six minutes old.
Murray, who was surprised to be starting the game, hit the third by running across the face of Vale's defence before snapping in a shot which took a deflection to confuse keeper Mark Goodlad on its way into the net.
When Brooker swept in his second goal from a low cross from the energetic Scott Brown on the left it made it 4-0 and the game looked all over.
Port Vale, who arrived unbeaten and second in the table, had other ideas.
Between the goal fest Nathan Lowndes and Michael Cummins had both drawn desperately late saves from City keeper Steve Phillips.
A home defence in which Matt Heywood was looking increasingly uncomfortable was being exposed for speed.
At one point Heywood needed an inhalant and didn't come out for the second half. But it was a mistake by his partner, Welsh international Dave Partridge, which raised Vale's game.
He gave the ball to Lowndes who steered it away from Phillips and into an empty net.
Minutes later Bell completed a move he began by sending in a downward header from George Abbey's accurate cross from the right.
It was game on again and Vale went on to dominate the second half aided by the absence of Brown who had limped off with a groin injury.
Only Phillips ensured City's two-goal lead remained intact to avoid a tense finale for Robins' fans.