Norwich went into the clash second and on the back of an eight-match unbeaten run in the league, but it proved a frustrating fixture at a packed Carrow Road for Alex Neil's side, with an Alex Tettey own goal the difference.
After Patrick Bamford saw his low cross sliced behind for a corner in the eighth minute, Tettey accidentally diverted the resulting set-piece beyond the helpless John Ruddy to give Middlesbrough the lead.
Daniel Johnson put Preston ahead after 12 minutes, but Michael O'Connor eventually restored parity early in the second period via a penalty.
A Richard Duffy red card for handball inside the area seven minutes later allowed Joe Garner to convert a penalty of his own, though the day's third spot-kick ultimately cost Preston two points five minutes from time.
John Welsh brought O'Connor down and the midfielder slotted away his second of the day, securing a draw for Vale and leaving Preston four points ahead of third-placed MK Dons.
