Alex Jones' first-half goal was all that Port Vale needed to see off MK Dons 1-0 and end a winless run away from home that stretched back to April.
AFC Wimbledon's win at Oxford earlier in the day saw Neal Ardley's side go above MK Dons for the first time in the English Football League and Karl Robinson's day got even worse when Jones netted the winner.
The hosts started brightly and Kieran Agard's downward header was well saved by Jak Alnwick in the Vale goal.
But little more than a minute after watching his strike smack against the Port Vale post, Dons defender Dean Lewington's misjudged handball handed Bruno Ribeiro's side a penalty 10 minutes before the break.
And although Jones' initial strike was superbly saved by Dons keeper David Martin, the Birmingham loanee was perfectly placed to smash home the rebound.
Substitutes Nicky Maynard and Brandon Thomas-Asante both forced saves from Alnwick late on and Ben Reeves hit the bar with a free-kick but they were unable to grab an equaliser.
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