The Spireites’ Good Friday battle at the home of fellow relegation strugglers Vale ended 2-1 to the Valiants thanks to a late goal.
A below par Town side went a goal down on the stroke of half-time, before goalscorer Danny Pugh was red carded for a rash challenge on Jacob Brown.
The visitors levelled through Louis Reed’s free-kick but naive defending allowed Luke Hannant to win it on 79 minutes.
Elsewhere Grimsby picked up point to leave Town four points from safety, albeit with two games in hand over the Mariners.
It wasn’t the start Jack Lester wanted from his side and throughout the game they failed to match the performance levels set against Notts County in the previous outing.
Brad Barry’s afternoon could have ended after just two minutes, the right-back appearing to catch Vale’s Luke Hannant with an elbow as they competed for a high ball.
Referee Tony Harrington decided it was just a yellow card offence and Hannant was fine to continue after treatment.
That wasn’t the only early let off for Town, a David Worrall cross from the right inviting Tom Pope to head at goal, the big striker mercifully nodding the ball well wide.
Vale’s gameplan was to bombard the Spireites with free-kicks and corners but despite their aerial threat, they were met by stout defending in the early stages.
Having weathered pressure Town looked to break with pace and a Louis Reed long ball released Zavon Hines, the winger’s left-foot effort palmed onto the post by Ryan Boot.
Aaron Ramsdale was called into action twice in quick succession as Chesterfield failed to get tight to men cutting in from the flanks, Cristian Montano sending a shot straight at the keeper, Hannant forcing him into a fine save low to his left.
At the other end a Sid Nelson ball over the top was brought down beautifully from Hines who cut inside his man and shot tamely at Boot.
Although Vale’s attacks weren’t as regular or sustained the longer the half went on, they remained a threat when Pope got the better of his markers - Hannant shooting wide after one flick on, Tyrone Barnett somehow hitting the bar and not the net from a couple of yards out after another.
The half ended with two huge moments - the first a goal and the second a sending off, both for Vale, both for Danny Pugh.
They took the lead when a long ball was only cleared as far as Worrall who had all the time in the world to set himself and hammer across the face for Pugh to get a touch and beat Ramsdale.
Seconds later, Pugh appeared to leave his feet as he challenged Jacob Brown and referee Harrington wasted no time in producing his red card.
Having dominated possession agains the 10 men for the first 10 minutes of the second half, Town levelled the game in fine style.
Kellett was fouled right on the edge of the box to halt a flowing Spireites move, and Reed stepped up to float a wonderful free-kick past Boot and into the net.
Despite the man advantage, Town lacked patience in their attacking play in the wake of the goal and were caught out trying to force play on a couple of occasions, Cristian Montano streaking away on the break to win a corner before Vale countered again and Pope wasted a good chance on the penalty spot.
Chesterfield came agonisingly close to the lead when Barry fired in a wicked cross from the right and Dennis powered a header against the crossbar.
The tension boiled over on 78 minutes when Talbot and Worrall tangled in the corner, the Spireite appearing to push his man over, both players earning yellow along with Hines and Hannant for the shoving that ensued.
Former Vale hero Louis Dodds and Josh Kay made their entrance with 10 minutes remaining, Kellett and Hines exiting the game as Town went for the win.
But it was a home substitute who grabbed the vital next goal, Chesterfield allowing a ball to bounce and paying the price as Ben Whitfield went in between Nelson and Barry to curl a beautiful finish past Ramsdale.
Lester responded to the goal by sending on Chris O’Grady for the final minutes, in place of Brown.
There was a golden chance to level when Boot came for a ball and didn’t make it, Dennis attempting to find the empty net from 25 yards first time on his right foot but only sending it over.
Talbot got in behind the defence deep in stoppage time and could do little with the opportunity, Boot gathering his low shot gratefully in what was the last chance for the visiting side before the final whistle ended a bitterly disappointing afternoon.
Chesterfield: Ramsdale, Barry, Talbot, Whitmore, Nelson, Weir, Reed, Brown (O’Grady 86), Hines (Kay 80), Kellett (Dodds 80), Dennis. Subs: Anyon, Smith, Maguire, McCourt.
Port Vale: Boot, Raglan, Howkins, N. Smith, Montano, Worrall (Harness 89), Pugh, Tonge (A. Kay 54), Hannant, Pope, Barnett (Whitfield 57). Subs: Hornby, Davis, Gibbons,Turner.
Goalscorers: Pugh 45, Whitfield 82; Reed 56
Yellows: Worrall 78, Hannant 79, Smith 90; Barry 3, Talbot 78, Hines 79
Reds: Pugh 45
Referee: Tony Harrington. Assistants: Paul Hodskinson, Adam Matthews.