Vlasic was born in Rochester, Pennsylvania. He attended Center High School and was a standout in football and basketball there. He was the starting quarterback for Center in his junior year, throwing for 777 yards in the first eight games of the season. In his junior year of basketball, Vlasic was a starter, in one game scoring 23 points and 11 rebounds with a bad back. He continued to play football and basketball during his senior year, with performances that included a 36-point effort against Las Vegas Western High School, making 18 of 20 shots. As a result of his high school career, Vlasic was inducted into the Beaver County Sports Hall of Fame in 1999.