Louis Dodds’ strike six minutes into the second period appeared to be sending Vale up to eighth in Sky Bet League One but goals from Jamie Murphy and Chris Porter in the final seven minutes turned the game on its head.
Vale started well and almost took a fourth minute lead when Chris Lines cracked a shot against the base of a post.
And a fired-up Vale side continued to pose more of a threat as the strong runs of Ben Williamson stretched the visitors’ defence.
United saw more of the ball from the midway point of the first-half, but they failed to trouble home goalkeeper Sam Johnson.
And Vale took the lead five minutes after the interval when Adam Yates’ pass was deflected into the path of Dodds, who took a touch on the run before planting an unstoppable 14-yard shot which tore into the top left-hand corner of Mark Howard’s net.
The contest opened up after the goal with Stefan Scougall providing the brightest spark for the visitors and Vale threatening on the break.
With the game in its final stages, Murphy slotted the equaliser into the bottom corner of Johnson’s net from 12 yards after getting on the end of John Brayford’s low cross from the right.
And Murphy turned provider as the game approached stoppage time when his driven cross was placed low into an empty net from point-blank range by substitute Porter.