It was the hosts that had the better of the first half, but they couldn't find the net despite plenty of dangerous possession in the wide areas.
Boss Tony Humes made two changes to his starting line-up, bringing in Tom Lapslie for the injured David Fox and Gavin Massey for Dion Sembie-Ferris.
Prior to the game, the gap to safety was six, but it was reduced to four with the vital win at Vale Park as the U's extended their recent run to seven points out of an available nine.
George Moncur and then Jacob Murphy gave the U's a two goal lead and Doods late goal was just a consolation for Vale.
They keep on fighting and go into Barnsley on Monday in good spirits. The home side were on the front foot from the start, and U’s keeper Sam Walker was in action in the very first minute.
A dangerous cross from the left was met by Mark Marshall, but the header went straight at Walker.
Crosses were causing some discomfort for the U’s defence but on two or three occasions, the decision making from the players crossing into the U’s box was disappointing for the hosts.
On twelve minutes, an offside flag came to the U’s rescue but it did look like Tom Eastman was confident that Chris Birchall was beyond the last defender before he clipped a shot into the corner of the net.
A golden chance was wasted by the hosts four minutes later, with Louis Dodds completely miskicking when well placed fifteen yards from goal.
And seconds later, Wiliamson shot straight at Walker as the home team continued to ask all the questions.
The game was twenty seven minutes old when the U’s had their first effort on goal, as Jacob Murphy cut inside onto his right foot but could only find keeper Sam Johnson with his shot from the edge.
Walker in the U’s goal didn’t have to make any more saves in the first half, but he watched nervously as a low cross found space instead of a home forward on the half hour mark.
A shot was also blocked before it could reach him, and then he watched a shot from Dickinson whistle just wide of his left upright.
A break two minutes from the break saw the U’s create their second chance, but Murphy could only find the sidenetting from an acute angle.
Chris Porter had done well on the halfway line, before Gilbey and Moncur combined for the latter to have a shot palmed out to the Norwich loanee.
That was the final effort of the first half, and without any added time, the referee blew the whistle for the break.
Tony Humes would have wanted a bit more urgency and his side gave him exactly that from the off in the second half.
Gavin Massey cut inside but couldn’t find the target with right foot curler and Jacob Murphy shot wide from just outside the edge of the box.
Those two chances came in the first ninety seconds of the half, and the tide had definitely turned.
The Vale captain shot well over from a corner after Walker had palmed the set piece away but it was a rare attack forn the hosts.
On fifty four minutes, the U’s went ahead and it was no more than they deserved for their attacking intent.
Gavin Massey, Alex Gilbey and Chris Porter were all involved, before the latter found George Moncur just inside the box, but the former West Ham trainee still had plenty to do.
The pitch was soft underfoot and he had to dig the ball out of his feet before guiding a right foot shot past Johnson.
Gilbey shot wide and Massey over the bar, as the U’s continued to press and look for a second goal.
That came on seventy six minutes, with loanee Jacob Murphy scoring his second in three games since his move from Carrow Road.
He burst past a defender with ease and drilled his shot low and hard across the keeper and into the bottom corner.
Massey had his third shot of the game three minutes later but once again, failed to test Johnson as the U’s looked to increase their lead.
A goal from Dodds in the eighty fourth minute made for a nervous end but the only real opening of the closing stages came for the U’s, with Healey failing to regain a two goal advantage for the U’s.
The final whistle went after over four minutes of injury time, with the U’s picking up a vital three points in their battle to avoid relegation.
The gap to safety reduced from six to four, with Barnsley visiting the Weston Homes Community Stadium on Easter Monday.