Millers blown off course
After a goalless first half playing into the elements, the Millers could not make the most of the conditions after the break and failed to test goalkeeper Stuart Tomlinson enough.
The decisive moment came 12 minutes after the restart when Dale Tonge brought down Doug Loft in the area and Marc Richards converted the resulting penalty.
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Hide AdJohnny Mullins had an effort scrambled off the line late on.
The defeat was Rotherham’s third in four games and means they drop below Vale in League Two and further off the play-off pace.
Millers: R. Taylor, Tonge, Griffiths (Newey 64), Raynes, Mullins, Marshall (Williams 60), Schofield, J. Taylor, Wood (Harrison 75), Revell, Grabban
Port Vale: Tomlinson, Yates, Loft, McCombe, McDonald, Morsy, Griffith, Rigg, Taylor (James 85), Richards, Williamson (Dodds 79)
Referee: D. Mohareb
Attendance: 3,189