Chesterfield had only one effort on target but it was the one that counted as substitute Marvin Robinson earned Roy McFarland's team a vital three points.
Robinson settled a forgettable game with a close-range header on the hour, two minutes after replacing David Reeves.
Vale will be furious at the way they left Robinson in splendid isolation at the back post to convert Gus Uhlenbeek's right wing cross, but their overall display was hardly promotion material.
The first half was a shapeless affair with both teams sending passes astray, but Billy Paynter brought the game to life with a superb left foot shot on the turn from the edge of the area which brought an equally outstanding save from Carl Muggleton to tip the ball away.
That was the start of Vale's best period in the match and Steve McPhee almost punished a mix-up between Muggleton and Steve Payne in the 37th minute but headed wide.
The pace of Adrian Littlejohn unsettled Chesterfield's defence but Vale had little to offer in the way of a finish as the midfield failed to deliver a decent service.
The game appeared to be drifting towards stalemate until Vale went to sleep and Robinson punished them with a neat finish. The striker has yet to agree a long-term deal with the club but this was his sixth goal and Chesterfield desperately need him to sign up for the rest of the season if they are to have a chance of beating the drop.
After Robinson pounced, Vale flickered briefly with McPhee just failing to get a touch on a cross but manager Brian Horton knows his team have to improve on the road if the play-offs are to be a realistic target.