Chapchai played a three-under 68 for the final round, but Lascuna shot up the leader board to catch him at the end of regulation play thanks to a bogey-free 63.
The two were tied at a 10-under aggregate score of 274 at the Seri Selangor Golf Club outside Kuala Lumpur, forcing a playoff.
But Chapchai, whose victory Sunday is his fourth on the Asian Tour, clinched the win by rolling home a par putt on the first playoff hole after Lascuna stumbled to his first bogey of the day.
The tournament's first and second-round leader Anirban Lahiri of India finished in sole third, his final-round 68 unable to make up for a damaging five-over-par76 he shot in the third round.