Vale, the early doors leaders, showed just why they top the Second Division with an emphatic 1-0 win over City.
Teenager Billy Paynter's 71st minute volley - his fifth goal so far - won it for them but they were always the stronger side.
City had failed to clear the second of two corners and the ball broke to Paynter eight yards out. The celebrations angered Vale's manager Brian Horton who ran on to the pitch demanding his players re-started.
But the defeat, City's first in sixteen home games, left their manager Danny Wilson dejected as his side loses ground.
With both sides showing too much respect to each other there had been a watchful weary start of a match City felt they had to win.
The game was in it's 27th minute before it burst into life and then Vale's striker Stephen Brooker rose above the City defence to glance Paynter's left-wing cross just wide of the post over his right shoulder.
Stunned by this near-reverse, City hit back quickly with a tremendous shot from Christian Roberts. He was 25 yards out as he moved onto a ball slid forward by Louis Carey and his shot screamed just wide.
There was an even closer call for Port Vale six minutes later. Again Carey was the provider only this time Lee Miller thumped a header against the bar. It dropped down vertically but Vale scrambled it away to escape with a corner.
Although not a natural flanker the industrious Roberts was covering an impressive amount of ground for City but changes must have been in the minds of both managers at half time.
Late into the second period City had used their full quota of three but Horton stood by his starting side who were defending with great tenacity.
Late on, City's keeper Steve Phillips kept them in there with two fine saves and then in injury time Bristol had to thank two goal-line clearances by Carey for escaping a heavier defeat.