Wasteful Cobblers slipped to yet another League Two defeat as their poor start to the season continued .
Town were left to rue another first half of missed chances before succumbing to a sucker punch from Ben Whitfield prior to the break, and their job was made doubly difficult when Aaron Pierre was shown red in the second half for a second booking.
Emmanuel Oyeleke's second goal ensured the hosts grabbed all three points, leaving a dejected Cobblers side with just one win and no clean sheets to their name in the opening eight games of League Two.
It was a familiar tale, with chances missed after yet more talk earlier in the week that Town were rectifying their weaknesses on the training ground. Sadly their habit of losing, even when playing well, does not look like a habit to be easily broken.
In a first half packed with chances, it was the Cobblers who had the lion's share of the opportunities - with Sam Hoskins coming close early on with a volley from the edge of the area that just looped over the crossbar.
Hoskins then turned provider as he fizzed in a cross across the area which Andy Williams came within a whisker of nodding home, but the striker couldn't quite execute the timing of his diving header.
Former Cobblers loanee Oyeleke may only have played a handful of games under Aidy Boothroyd, but since his Sixfields stint he's beefed up, and he caused plenty of problems for Town in the middle of the park. He wriggled his way past Foley to force a low, if comfortable save, from Cornell.
Connell Rawlinson then came close for the hosts when he rose highest at the far post at a free-kick, but his header inched wide of the post.
But Town have been guilty of missing a host of gilt-edged chances this season, and the latest such occasion came when Williams should have bagged his first goal for Dean Austin's side.
The lively Hoskins broke clear again and played in Williams, who with a slight angle but just the keeper to beat, missed the target altogether. It was a missed chance that would be rued moments later when Vale broke the deadlock.
Whitfield cut inside from the right before unleashing a crisp shot that beat Cornell and nestled in the bottom corner. It was a fine strike, but he could arguably have been given a tougher time in getting the shot away.
Dutch striker van Veen then should have added to his tally of goals this season when he headed over from another Hoskins cross, despite being virtually under the crossbar.
The half-time talk in the stands will have been a similar conversation amongst Cobblers fans, how were they losing this game?
But a tough task became tougher when Aaron Pierre was sent off for a second bookable offence. Pierre had been untidy with his challenges for the majority of the afternoon, and earned a needless booking when he barged Tom Pope over instead of shepherding the ball out of play. That petulance proved costly when he hacked down last man Miller just moments later, leaving him to take the lonely walk down the tunnel.
From there it was always going to be a tough task for Town, who brought on Jordan Turnbull to try and shore things up at the back.
But the damage had already been done in the first half. Had Northampton taken any of their first half chances they may have had something to cling onto.
So when Oyeleke neatly curled in a second with 20 minutes left, it signified game over - with the crossbar coming to Town's rescue twice to stop the score getting uglier.
For Austin it will have been a familiar site from the sideline. Once the losing habit sets in - as it has done for a large percentage of this squad over the last year - it's difficult to get out of it.
Cobblers: Cornell, Facey, Taylor (c), Pierre, Buchanan, Foley (O'Toole, 82), Crooks, Hoskins, Bridge (Turnbull, 61), Williams (Waters, 57), van Veen
Subs not used: Ward, Odoffin, Whaler, Bowditch.
Port Vale: Brown, Rawlinson, Smith, Legge, Clark, Whitfield (Worrall, 82), Oyeleke, Conlon (Joyce, 67), Hannant, Miller, Pope
Subs not used: Hornby, Quigley, Kanu, Vassell, Hardcastle.
Referee: Thomas Bramall