Substitute Ben Williamson broke Forest Green hearts as his late winner saw Port Vale edge home at The New Lawn.
Ashley Vincent put Vale ahead early on as he skipped past two challenges before coolly finishing and then set up Clayton McDonald for the second.
James Norwood curled a shot into the top corner to give the home side hope before skipper Eddie Oshodi equalised.
Matty Taylor had a shot cleared off the line for Forest Green before Williamson headed home the winner.
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Forest Green Manager Dave Hockaday told BBC Radio Gloucestershire:
"I'm proud as punch of the lads but we aren't managing the moments well enough and that's what we're getting punished for. Fair play to them for taking the chances.
"In the second half I thought we were outstanding, and there was only one team that was going to win the game.
"We've had shots cleared off the line, we've missed chances andwe've conceded the softest of goals. We can't keep on shooting ourselves in the foot.
"After today's performance you'd expect to come away with something but one silly moment, we lose the game."