Former Valiant Lee Collins provided a crucial own-goal as the Vale came from behind to secure a point and all but secure promotion to League One.
In front of a 12,000-plus crowd, the Vale looked lethargic and had an awful start as Clark Carlisle scored after just three minutes.
Vale equalised when Liam Chilvers, having one of his best Vale games, headed in Tom Pope’s cross just before the break.
But Northampton took a deserved lead through O’Donovan before Collins’ sliced clearance saw Vale level.
As news of results elsewhere filtered in, Vale fans celebrated at the end knowing that only an unlikely 28 goal swing in favour of Cheltenham on the final day will stop them going up.
Vale: Neal, Yates, Purse (Shuker), Chilvers, Jones, Myrie-Williams, Griffith (Morsy), Dodds (Birchall), Loft, Hughes, Pope
Subs: Johnstone, Duffy, Lloyd, McCombe