Marc Richards scored twice in the first half as Port Vale beat out-of-form Rotherham to leapfrog the Millers into the play-off places.
His opener came from a 20-yard free-kick which slipped through keeper Conrad Logan's hands and into the corner of the net.
And he headed home his second from eight yards out after Tom Pope headed on Gareth Owen's long clearance.
Rotherham's Lewis Grabban had an effort ruled out for offside.
His side have now won just one from six.
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Port Vale boss Micky Adams told BBC Radio Stoke:
"We looked as though we had a resolve and a determination to get a clean sheet and not to concede today, which was very pleasing.
"After 15 minutes we got on top and got the two goals, and that was really the only bit of quality forward play that we had all day.
"I looked at the boys and they looked like they wanted to attack the ball - and they did that well."
Rotherham boss Andy Scott told BBC Radio Sheffield:
"I thought we were the better side and we lost. But we lost because we made two individual errors which you just can't make in this league.
"The two goals were just, from my point of view standing there watching it, absolutely horrific really.
"The first goal was a really poor error and the second one is something which we've done six days this week and people just haven't done their job."