Seconds after escaping what looked like a stonewall booking, Justin Cochrane threatened to break the stalemate in the 14th minute when he found acres of space 25 yards out but his shot lacked direction.
Port Vale turned on the style in the 23rd minute after doing little else before when Stefan Moore's back-heel on the edge of the box set up Richie Humphreys for a volley that zipped over the bar.
The Valiants started to command more possession as the half went on but struggled to turn it into goals with Neil Cutler being given another comfortable save from Danny Whitaker after 36 minutes.
Things heated up five minutes before the break when, after a seemingly harmless nudge by Richard Walker on Dave Hibbert, the former Vale striker looked to spit in the defender's direction.
Rotherham almost grabbed a goal in first-half stoppage time when Scott Wiseman delivered a great cross but Will Hoskins' header was well caught by Mark Goodlad down to his right.
The Millers took the lead only two minutes into the second half when Michael Keane's curling cross from the left found the head of Cochrane and he sent the ball looping into Goodlad's top left corner.
The visitors made it a quick-fire double in the 53rd minute when substitute Dave Worrell picked out Hibbert at the back post and he climbed above his marker to put another header into the Vale net.
The home side were given a lifeline in the 74th minute when Scott Wiseman was caught pulling the shirt of Walker inside the area by eagle-eyed referee Darren Drysdale.
Despite still recovering from a stomach virus, substitute Leon Constantine stepped up to the spot and confidently stroked the ball into the bottom left corner of Cutler's goal.
But Goodlad cost his side the chance to get back into the game when he unnecessarily chased a loose ball on the edge of the box only to be caught out by Hoskins who finished brilliantly from a tight angle.