Vale slipped to their sixth defeat in a row thanks to a last-gasp winner by former Valiant Calvin Andrew.
Rob Page had rung the changes bringing in Chris Robertson, Richard Duffy, Tom Pope, Chris Neal and Tommy O’Sullivan.
The first half presented few real chances for either side as it ended goalless. Tom Pope was injured in the 47th minute and replaced by Williamson.
The Valiants looked to be heading for a valuable point only for Rochdale sub Andrew to head home with practically the last effort of the game to snatch all three points for the home side. It was a bitter pill for Vale to swallow and it means Vale are slowly being drawn more and more into the relegation battle.
Vale: Neal, Veseli, Duffy, Robertson, Dickinson, Marshall, Brown, O’Sullivan, Birchall (Daniel), Dodds, Pope (Williamson)
Subs: Coulibaly, Yates, Johnson, O’Connor, Campion