Having represented Netherlands U19, Vlaar was called up to the Netherlands U20 squad for the 2005 FIFA World Youth Championship took place in the Netherlands between 10 June and 2 July 2005. Vlaar, reserve captain behind Hedwiges Maduro, was an important player in the Dutch team and was active in every match the team played on the tournament. Netherlands U20 had a smooth run through the group stage, winning their three matches respectively easy against Japan U20 (2–1), Australia U20 (3–0) and Benin U20 (1–0). After beating Chile U20 (3–0) in the next round, the Netherlands U20 reached the quarter finals against later finalist Nigeria U20, which they lost 10–9 on penalty shootout) following a 1–1 draw at Extra time. Despite Vlaar's equalizing goal and successful penalty kick, the Netherlands U20 didn't advance to the next stage.