Both sides hit the woodwork in a frenetic final five minutes as Chris Zebroski cracked an angled drive off the Vale post in the 85th minute.
On the counter attack, Vale sub Sean Rigg smashed a long-range effort off the face of the United bar.
United central defender Chris Robertson then had a goal ruled out by the assistant referee as stoppage time approached.
Recalled Billy Kee fired an early warning shot just wide, which spurred the Valiants into waves of attacks.
Home skipper Lee Mansell made a superb block to deny Vale's Robert Taylor as he let fly from the left and at the other end Valiants right back Adam Yates blocked a goal-bound shot by Seagulls forward Jake Robinson.
Zebroski came closest to scoring in the first half after shrugging off the attention of three Vale defenders and crashing a thumping drive just wide from the right.
As half time approached, Eunan O'Kane also went close with an angled drive from the right.
United began the second half as they finished the first, in the ascendancy and on the front foot.
Kevin Nicholson's corner from the right was met with a downward header by central defender Guy Branston but the ball flew wide.
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