It was a routine substitution by manager David O'Leary which changed the game after Villa had struggled to break down Port Vale.
Phillips' introduction for the out-of-form Juan Pablo Angel had an immediate effect as the former England striker's first touch put Milan Baros clear to opening the scoring after 70 minutes.
Baros was on the mark again four minutes later with his eighth goal of the season but it was not a memorable tie in any shape or form.
For the opening 70 minutes Port Vale's gallant giant-killing aspirations were kept alive by a combination of good defensive work and Villa's lacklustre play
But it was Baros who delivered the goods after the Czech international had experienced a frustrating opening 60 minutes.
Even in the early stages of the game it had become obvious that there was not going to be a repeat of the last Cup meeting between these two teams when Villa ran out 6-0 winners in the 1990 fourth round tie.
Unlike on that occasion they lacked the killer instinct and plucky Port Vale were able to show commendable fortitude with a compact defensive performance.
Instinctively Villa dominated the game from the start with Vale content to frustrate their Premiership opponents.
But Villa alarmingly struggled to break the resilient Vale rearguard in which George Abbey created an early impression with some solid defensive work.
The worrying aspect of the game was Villa's inability to step up a gear against their lowly opposition who relied on the spasmodic break to hit back.
With Villa unable to turn their superiority into goals an anxious air developed and there was every reason for Villa to be concerned when Michael Husbands produced a right-wing cross which Sam Togwell headed just wide with goalkeeper Thomas Sorensen at panic stations.
Villa's response was an individual effort from Baros when Vale's goalkeeper Mark Goodlad saved with his feet.
Seconds later in the high wind the frustrated Baros blasted a shot aimlessly over the bar.
The lack of urgency on the part of Villa played very much into Vale's hands with George Pilkington and Clayton Fortune both performing to the highest standard.
With Villa so much on attack Vale posed problems on the break with Husbands, a former Villa player, heading into the arms of Sorensen from close range and seconds later Leon Constantine collected a deflected pass from Mark Innes. but with only the Villa keeper to beat lashed his shot on the run wide of the post.
But the introduction of Phillips set up Baros for his opening goal and four minutes later Olof Mellberg headed a James Milner corner to Baros to score from close range.
In a stirring finish Vale got back into the game when Nathan Lowdes headed home a free-kick from Craig James.
Fellow substitute Sean Doherty should have equalised but three minutes from time a mis-hit shot by Lee Hendrie was headed home by Steven Davis.