Bradford's promotion hopes suffered a massive blow as Port Vale inflicted a fourth defeat in 11 days on City.
Vale's Paul Marshall intercepted a pass in midfield and evaded Matt Clarke's tackle before passing to the unmarked David Howland who scored from 10 yards.
City dominated possession but were made to pay for missing numerous chances.
Michael Boulding had a close-range effort blocked, Graeme Lee headed over from six yards and Steve Jones saw a drilled effort saved as Vale held on.
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