Goalkeeper Leo Fasan saved his second penalty in three games but to no avail as Vale’s winless streak stretched to seven games.
Manager Michael Brown named an unchanged starting eleven but named himself as a sub.
It was an even opening but low on quality although the home side did have the better chances. However, on 26 minutes, more poor defending from the Valiants saw Aiden O’Brien make no mistake to open the scoring. There were further opportunities for Onyedinma and O’Brien as the home side looked to double their lead.
Meanwhile, Vale suffered a big blow on the half-hour mark as Remie Streete limped off to be replaced by Scott Tanser. The Valiants were now under some pressure and were grateful for some good handling from on loan goalkeeper Leo Fasan. Vale were reduced to a speculative shot from Danny Pugh as the first half finished.
Early in the second period Kjell Knops conceded a penalty but once again Fasan pulled off a fine save. However, just like the Peterborough game, Vale then conceded a goal seconds after the penalty save as Cooper scored from the resulting corner.
Vale then lost Anthony Defreitas to injury and were thankful for another couple of fine saves from Fasan as Millwall looked to extend their lead. The Valiants were looking disjointed and with the exception of Fasan there were generally lacklustre performances on show. The home side looked the most likely to score but Fasan kept them at bay, but Vale now face a crucial home game against Bristol Rovers as their terrible away form continued.