Vancsik had six birdies and one eagle on the Real Sociedad Hipica Espanola Club de Campo course. Jose Filipe Lima is a stroke behind.
Martin Kaymer and Steven O'Hara are two strokes behind the Argentine, and Emanuele Canonica and Gary Lockerbie are three back.
Teenager Roy McIlroy (73) was tied for 43rd.
Play was halted because of poor light with three players still on the course.