Blues have picked up seven points from their opening three games following their return to the second tier.
They also beat Sheffield United in the EFL Cup first round, while Darren Moore’s Valiants are yet to win in the league this season.
Birmingham boss Chris Davies made 10 changes to the side which beat Oxford United on Saturday and it was two debutants who created the best early chance.
Marvin Ducksch linked up with Lewis Koumas and the on-loan Liverpool teenager shot from a tight angle past the near post, before seeing another effort go wide.
Cameron Humphreys had a shot deflected over the bar at the other end from a free-kick as Port Vale started to push forward. It was from set-pieces where they looked most threatening, coming close from a number of corners.
Blues keeper James Beadle made two diving saves to deny Humphreys’ header and one from Mo Faal while Croasdale also had a chance that went wide, before Headley finally found the net.
The ball bounced out of the box and when it was crossed in a second time by Croasdale, Headley’s diving header from close range this time did not miss.
Forward Jay Stansfield was brought on as one of three changes at the break for Birmingham but they were unable to muster a shot on target in the whole game.
Curtis was found in space as the best chance again fell to Port Vale, and though he sent Beadle the wrong way his effort was off target.
Birmingham piled on the pressure late on as expected, but Port Vale were able to close out an impressive win.