A penalty and a sending off contributed to United suffering a first away defeat in Sky Bet League One since early October.
Chris Basham was sent off in the first half for a foul on Michael Brown in the Boxing Day clash at Rob Page's Port Vale.
Ben Williamson gave Vale a first half lead, Jamal Campbell-Ryce levelled for the ten-men before Dany N'Guessan netted a late penalty.
Chief Clough made three changes for the trip to Vale Park. Cuvelier, Campbell-Ryce and McGahey started, whilst Flynn missed out through injury and Baxter and Adams dropped to the bench. Port Vale's starting line-up included former Blades favourite Michael Brown.
The early action was played at a tempo but chances were limited. A Harris effort was blocked and Marshall's shot was deflected wide. United were looking the more dangerous, particularly through Scougall, but Vale took the lead on 16 minutes following a disputed corner.
Vesili's short delivery caught the Blades on the back foot and WILLIAMSON's clip towards goal beat Howard, who had to contend with Duffy's presence in front of him.
The Blades hit back and the next 15 minutes was all pressure from the visitors. Scougall had an effort blocked by Duffy and then the diminutive Scotsman beat two players and watched on as his shot took two deflections before goalkeeper Neal made a tremendous reaction save. Vale could only half clear and from the second phase, Murphy delivered a cross but Cuvelier ended up in the net instead of the ball.
If Neal excelled himself with the previous save he did even better on 28 minutes, this time great build up play between Murphy and O'Grady presented a shooting opportunity for Scougall, but Neal dropped to his right to push the ball around a post.
Just after the half hour, McGahey cleared off the line a header from O'Connor and there was another chance for the Blades, Basham heading over a Campbell-Ryce cross before the drama began.
Ex Blade Brown had already gone through the back of Scougall and sliced down Murphy before writhing around in apparent agony following a collision with Basham. The Port Vale players surrounded the referee and Basham was shown a straight red card by referee Attwell. Brown, after treatment, was OK to continue and managed help his Vale team-mates through to half-time with their lead intact.
The Blades made a change at the break with McCarthy replacing Cuvelier and the visitors won two corners in the opening 90 seconds after the interval. Also early in the half there was half-hearted penalty appeals at both ends and Dodds headed wide from a corner.
With the wind and rain, as well as the cold, causing problems, United were matching the home side, despite the numerical disadvantage, emphasised by the equalising goal on 55 minutes. Murphy skipped away from two challenges on the left and his pull back was perfect for CAMPBELL-RYCE to slot past Neal.
Shots from both sides, Scougall and Brown, were at the goalkeepers as the weather deteriorated. Good work from Scougall helped Murphy get a shot away but his effort was deflected wide and Reed replaced Scougall, who hobbled off.
As the snow began to fall, Harris did well to keep out N'Guessan following a corner but the Blades left back was adjudged to have fouled Marshall following a drag back which presented the home side with a penalty on 79 minutes. N'GUESSAN took responsibility and netted, despite Howard getting his hand to the spot-kick.
McNulty was thrown on late on and the Blades saw a lot of the ball but were unable to threaten Neal's goal in front of the 2,594 Unitedites. Four minutes of injury time saw just Brown shoot wide and a Doyle effort blocked before the final whistle signalled a Boxing Day defeat.
Port Vale: Neal, Yates, Veseli, Duffy, Dickinson, Marshall, Brown, O'Connor, Dodds (Lines 74), N'Guessan (Daniel 84), Williamson.
Unused Johnson, Robertson, Birchall, Nimely, Campion.
Blades: Howard, Harris, Basham, Doyle, Murphy, Scougall (Reed 71), McGahey (McNulty 84), McEveley, Campbell-Ryce, O'Grady, Cuvelier (McCarthy 45).
Unused: Baxter, Higdon, Turner, Adams.