However, Porter was on hand in the dying stages to snatch the victory and stun the home supporters.
The best chance of the opening half fell to Chris Lines after just six minutes, who saw his bid to open the scoring rebuffed by the frame of the goal.
Micky Adams's men broke the deadlock on 51 minutes through Dodds, who finished from close range to fire Vale ahead.
United pressed for the equaliser and found one with eight minutes remaining through Murphy, who tapped home from close range.
With the encounter ticking towards its conclusion, Porter pounced and fired home a dramatic winner.