John Durnin's deflected first-half injury time winner earned Vale three vital points in their bid to avoid the dreaded drop, but sent struggling Mansfield to their second home defeat in a week and almost certain relegation.
Durnin's 20-yard strike was deflected past stranded keeper Kevin Pilkington by his own player-manager Keith Curle after an even first half.
Mansfield, riding their luck as they chased a second-half equaliser, then squandered a hatful of chances to earn a point.
Craig Disley and Wayne Corden were denied by fine Mark Goodlad saves, while substitute Craig Mitchell, Disley and Bobby Hassell finished tamely with just the keeper to beat.
At the other end Vale continued to carve out the better opportunities. Steve McPhee and Adie Littlejohn wasted one-on-one chances to beat former Vale keeper Pilkington and then in the final minutes substitutes Steve Brooker and Ian Armstrong saw fierce shots somehow blocked by the keeper.
Vale, missing three players through flu, had survived an early Mansfield onslaught, Junior Mendes just failing to touch in Iyseden Christie's low cross and then Christie's close range, stabbed finish blocked in the six yard area.
Then Disley's precise header found Christie unmarked, but his cross-shot was beaten away by the busy Goodlad.
The Stags were twice given a let-off when Marc Bridge-Wilkinson curled long range drives against the woodwork with Pilkington well beaten.
But the home side still dominated the early moments and Adam Eaton's 35-yard piledriver forced Goodlad into a stunning, fingertip save.
From then on Vale dominated. In the 35th minute they somehow contrived to miss two gilt-edged chances. First Durnin's pass saw McPhee out-muscle Eaton, only to be blocked by Pilkington's legs.
Then from the follow-up Littlejohn missed from six yards, his header bouncing into the ground and up and over the bar.
Afterwards dejected Mansfield boss Curle said: "The players responded magnificently to last week's five-goal mauling. We did everything bar put the ball in the net." Vale manager Brian Horton was elated: "We created so many good chances for an away side, and probably scored from the hardest one of all."