 In the 2004–05 season, Vlaar made his unexpected Eredivisie debut at the age of 20.Due to suspensions within the first team squad at the end of the season, Vlaar was paired up with the experienced centre back Barry Opdam on 23 April 2005 in the away match against RKC Waalwijk (2–1).Five days later, on 28 April 2005, Vlaar made his European debut.