Millwall's slim hopes of making up ground in the race for the Play-Offs all but vanished as goals either side of half-time from Danny Whitaker and Akpo Sodje sent The Lions sliding to a second successive defeat.
Despite dominating long spells of the game, and playing the last 10 minutes against 10-men after Vale left-back Jason Talbot was sent off for stamping on Ryan Smith, Millwall just could not find that killer touch in front of goal.
It ultimately proved to be an afternoon of immense frustration for Willie Donachie's charges who can now put any promotion aspirations on hold for 12 months.
The Lions had to sustain an early period of Vale pressure as Sodje's third-minute deflected shot was held by Lenny Pidgeley and The Lions keeper then did well to deal with Danny Sonner's long-range effort.
Neil Harris created the visitors first half-chance on goal after seven minutes when his cross-shot from the left fell agonisingly short of Neal Ardley, who slipped as he was running on to try and meet the ball.
But it was the home side who enjoyed the upper hand in the opening exchanges and within seconds of Pidgeley again coming to the rescue to deny Sodje on 10 minutes, Port Vale had taken the lead.
A quick break saw the ball worked out to Leon Constantine on the right and the former Lions' striker delivered a delicious cross that was hammered into the roof of the net by Whitaker from 10 yards.
But instead of grabbing hold of the game, Vale seemed content to sit back and allow Donachie's charges to work their way back into proceedings - and that they did commendably.
Other than a couple of hairy-scarey quick breaks that caused a flutter or two amongst the visiting fans behind the go2 telecom stand - the remainder of the opening half was all Millwall's.
Mark McGregor did well to hack away a Danny Senda cross, Alan Dunne's 17th-minute header was somehow clawed away by keeper Joe Anyon and then Darren Byfield's shot on the turn flew wide as Donachie's men started to ask questions of the home defence.
Harris twice went close, first with a stinging shot that was deflected to safety and then six minutes before the break when, following a neat interchange with Byfield, the striker's sizzling drive smacked the top of the bar.
Some slick passing between former Vale Park favourite Dave Brammer and Byfield set up Dunne a couple of minutes later, but the midfielder's left foot effort curled wide of the right hand post.
The Lions boss brought on Poul Hubertz after the break for Ardley, with Dunne switching to the right flank and Harris operating on the left. That move nearly paid off on 51 minutes when a dinked Harris delivery was half-cleared, Tony Craig sent the ball straight back in and Dunne slid to connect with the ball, but didn't get enough power on the cross to trouble Joe Anyon in the home goal.
From the keeper's subsequent huge, booming clearance upfield the bounce of the ball caught the Millwall rearguard square enabling Sodje to send a looping header over the advancing Pidgeley.
That should have been game over - but Millwall simply refused to accept that. The introduction of livewire loan winger Smith added pace, purpose and penetration to the attack as Donachie's men created chance after chance in a purple patch during which they did everything but score.
Harris headed just wide from a Smith cross, Hubertz was twice denied by brilliant defensive blocks from McGregor, Craig's effort was cleared at the far post as the home defence were wobbling.
But when Hubertz curled over and then Harris' shot was somehow palmed away as it was about to squirm in at the near post, you sensed that the inability to convert any one of those numerous chances into a goal was going to prove costly.
That proved to be the case and ironically it was when Martin Foyle's side were reduced to 10 men that Vale managed to successfully regroup and sort themselves out collectively.
Despite an unsavoury melee that followed a deserved red card for Talbot's knee into Smith's head - and the Vale man's reluctance to leave the field - order was eventually restored by referee Murphy.
Even the most optimistic among the 283 travelling fans could not have imagined that a two-goal deficit could be hauled back in four minutes added time, and that proved to be the case.

Port Vale: Joe Anyon, George Pilkington, Jason Talbot, Danny Sonner, Leon Constantine, Danny Whitaker, Mark McGregor, Akpo Sodje, Robin Hulbert, Rhys Weston, Malvin Kamara (Colin Miles, 83mins). Subs not used: Paul Musselwhite, Nathan Lowndes, Michael Walsh, Ross Gardner.
Bookings: Hulbert (44mins), Talbot (62mins), Constantine (80mins)
Sending off: Talbot (80mins)
Scorers: Whitaker (11mins), Sodje (52mins)

Millwall: Lenny Pidgeley, Danny Senda (Chris Hackett, 86mins), Tony Craig, Marvin Elliott (Ryan Smith, 57mins), Paul Robinson, Richard Shaw, Alan Dunne, Dave Brammer, Neil Harris, Darren Byfield, Neal Ardley (Poul Hubertz, 46mins). Subs not used: Chris Day, Nathan Ashton.
Bookings: Harris (62mins), Hubertz (80mins), Byfield (80mins), Brammer (90mins)

Ref: M. Haywood
Att: 3,973 (Away 283)