A dull encounter at the National Hockey Stadium looked to be heading towards a goalless draw as both sides struggled to find the breakthrough in this League One encounter.
But Port Vale finally managed to break the deadlock on 69 minutes thanks to a thumping strike from Lee Matthews.
Their lead lasted only three minutes, however, as the Dons responded in the best possible fashion as Platt headed home brilliantly from a Nicky Rizzo cross.
The goals came as great relief to spectators inside the stadium who had watched a tedious opening hour.
It took 18 minutes for the first sight on goal of the afternoon to arrive and when it did Wade Small failed to deliver the goods as his shot flew over the bar.
Billy Paynter put a chance over the bar for Vale and Dean Lewington shot just wide for Milton Keynes, but it was Vale striker Matthews who missed the best chance of the opening period.
On 38 minutes he met Craig James' corner but headed inches wide of the far post.
Small was presented with another good chance after the break but once again failed to hit the target.
And the match finally burst into life on 69 minutes when the visitors took the lead after Paynter won a free-kick 25 yards from goal.
James set up Matthews with a short tap before the striker thumped an unstoppable drive past MK Dons goakeeper Matt Baker.
However, their lead was cut short within two minutes following a well worked move by the Dons.
The ball found its way out to substitute Rizzo on the left wing and his perfect cross was met by the diving head of Platt to level the scores.
Vale's Dave Hibbert was stretchered off in the final minutes with his head in a brace as they game ended on even terms.