The Reds are languishing in 19th in the table after picking up just a solitary point in three games in a week.
A 5-3 home loss to MK Dons was followed by a 0-0 draw at Fleetwood, but any optimism that brough disappeared with defeat to a Vale side who ended a run of six successive losses.
Mark Marshall scored the decisive goal late in the match.
Tykes goalkeeper Ross Turnbull had a busy afternoon but he could do nothing about Michael O’Connor’s free-kick.
The in-form Conor Hourihane levelled from the penalty spot after he was bundled over
The hosts had a penalty of their own but Jordan Slew fired over the bar.