Vancsik, who has held at least a share of the lead after the first three rounds, completed 54 holes at 13-under-par 203.
Mads Vibe-Hastrup posted a 5-under 67 and is alone in second place at 11- under-par 205. Alejandro Canizares, who shared the second-round lead with Vancsik, managed an even-par 72 and was joined in third place at minus-9 by 18-year-old Rory McIlroy.
'I know that Sergio won his first tour event in his fourth start and this is only my third as a pro,' McIlroy said. 'You never know what can happen, if I can shoot another 66 tomorrow you never know what can happen, but I am just trying to take it one day at a time and finish as high as I can.'
Vancsik opened his round with a birdie on the second, before he stumbled to a bogey on the third at Club de Campo. He atoned for that mistake with back-to- back birdies from the fifth.
The 30-year-old Argentine birdied the ninth and followed with another birdie on 10 to extend his lead to four over Gonzalo Fernandez-Castano, who struggled down the stretch.
Vancsik birdied the 12th and 13th, but gave those strokes back as he double-bogeyed the par-3 15th after finding water short of the green. He dropped another shot on 15 and his lead dipped to two over Vibe-Hastrup.
For the third straight day, Vancsik birdied the par-5 16th. He followed with a bogey on 17, but closed with a birdie at the last to push his lead back to two.
'I would have taken a 68 so I can't get too angry about the 14th,' said Vancsik, who won the Madeira Islands Open earlier this year. 'The 14th was playing really easy today, but I still hit the ball into the water. I made another silly bogey on the 15th as well from the middle of the fairway, but I am leading by two and happy going into the final round.'
Vibe-Hastrup birdied the second to go with eight front-nine pars. Around the turn, the Dane made his move.
He ran off three birdies in a four-hole span from the 10th to jump to 10 under and within three of Vancsik's lead. Vibe-Hastrup birdied the 16th, but faltered to a bogey on 17. He closed with a birdie at the last like Vancsik.
Marcus Fraser fired a 7-under 65 to jump to 8-under-par 208. He was joined in fifth place by Gregory Bourdy, Santiago Luna, Manuel Quiros and Jean Van de Velde.
Fernandez-Castano was done in by a triple-bogey on the par-5 16th. He stands alongside Emanuele Canonica, Christopher Hanell and Gary Lockerbie at minus-7.