Vlašić had an early start in international competition: she competed at the inaugural World Youth Championships, finishing eighth, and represented her country for the first time at the 2000 Sydney Olympics. Although the Olympics showed that she was not ready to compete at the senior level, she proved herself to be more than proficient at the junior level by winning the 2000 World Junior Championships with a jump of 1.91 m. She was a regular competitor at senior athletics meetings and was steadily improving, qualifying for further top-level senior events. Vlašić finished sixth at the 2001 World Championships in Edmonton with a mark of 1.94 m, a result which led the IAAF's Ed Gordon to mark her out as a future star in the event. She rounded off the year by winning her first senior gold medal at an international tournament, taking first place at the 2001 Mediterranean Games.