The game came to life with four chances in the space of two first-half minutes.
First Chris Birchall lashed in a left-foot shot which Rotherham keeper Adam Collin blocked with his legs after 19 minutes. Then at the other end a Wes Thomas shot was tipped over by Sam Johnson.
From the resulting corner Thomas' header was again tipped over the bar before Craig Morgan cleared Chris Robertson's header off the line from Birchall's corner.
In the 44th minute the home side were reduced to 10 men when Richie Smallwood was shown a straight red card by referee David Coote for a late challenge on Billy Knott.
Rotherham made two changes at the break, Rob Millsom replacing Nicky Adams with Agard on for Haris Vuckic.
Agard's first touch after just 30 seconds on the field was to head a Thomas cross just wide.
And after numerous other near misses including an Alex Revell header, the third Millers sub of the afternoon, on 78 minutes Vale failed to clear a cross from the left and the lurking Agard finally beat Johnson with a fright foot shot.