The Argentine stands at 14-under-par 202 through 54 holes. Vancsik, who has never held a 54-hole lead on the European Tour before this, will go for his first tour title on Sunday.
'This is the first time that I have had such a good chance to win on the European Tour,' Vancsik stated. '(Sunday) I will have some nerves because I know that if you lose your mind on this course then you can forget it.'
Mads Vibe-Hastrup matched Vancsik's 68 to move into second place at 10-under-par 206. Christian Cevaer managed just an even-par 72 to slip one spot into third at minus-9. He is one stroke clear of Euan Little (69) and 1995 champion Santiago Luna (72).
Early on, it was a three-way race between Vancsik, the second-round leader, as well as Luna and Cevaer. Those two both birdied No. 3 to grab a share of the lead at minus-10.
Vancsik carded his first birdie on the fifth to regain a lead which he would not give up the rest of the round. That was his third birdie in three days on the fifth at Santo da Serra Golf Club.
The 30-year-old Vancsik pushed his lead to two strokes with a birdie on the par-5 seventh. He parred three straight around the turn, while Cevaer moved back within one thanks to a birdie on No. 8.
Vancsik birdied the par-5 11th to again lead by two at minus-13. He extended that lead to threes strokes as he birdied the par-4 14th. Vancsik parred out to cap a bogey-free round. He saw his lead grow thanks to late bogeys by Cevaer.
'It is a surprise that I feel so good about the game so soon after changing my coach,' admitted Vancsik. 'I played well last week but missed the cut. This week I have played very well, but I did not expect to be in this position after changing so much in my game recently.'
Vibe-Hastrup, who was playing two groups ahead of the leader, got off to a quick start with birdies on one and four. Those birdies got him to minus-8.
The 28-year-old Dane parred six in a row from there. Vibe-Hastrup collected the third birdie of his round on the 11th. He birdied 15 and 17 to move to 11 under, but dropped a shot at the last to end four back.
'I am playing really well so that is one positive for me and I can take confidence from that,' Vibe-Hastrup said. 'I have to try and put pressure on him if he makes mistakes, and hopefully I can enjoy it as well. This is the first time I have been in the last group for the fourth round of a European Tour event.'
Cevaer birdied the third and eighth, both for the third consecutive round, to get to 11 under. That put him one behind Vancsik. However, he parred seven in a row before back-to-back bogeys from the 16th dropped him to minus-9.
Gregory Bourdy (68) and David Frost (72) share sixth place at 7-under-par 209. Alexander Noren, Fredrik Andersson, Robert-Jan Derksen, Jamie Donaldson and Simon Nash are one stroke further back at minus-6.
American Notah Begay III shot 3-over 75 and is now 11 off the pace.