As a youth player at KSK Beveren, Geert was under the influence of the great goalkeeping coach Vic Behiels. Behiels had previously trained such well known goalkeepers as Filip de Wilde and Jean Marie Pfaff. In Beveren Geert started his playing career, and in 1995 he was recruited by Brussel giants Anderlecht. For 5 years he struggled to become the number one choice, battling with de Wilde who was the obvious choice. In the end Geert went to Dutch Eredivisie side Willem II, where he showed some outstanding performances.
After De Wilde's disastrous performance in EURO 2000, Geert got his chance in the Belgian national team and was showing improving skills and abilities. This was not only recognised in Belgium and Holland, but also in the rest of Europe. De Vlieger became first choice, and an occasional captain, for his national side during their successful World Cup 2002 qualifying campaign, playing in all four of their finals appearances in the Far East.
The experienced Belgium goalkeeper was signed on a free transfer by Kevin Keegan in June 2004. As regular for the Tilberg outfit, the Aalst-born custodian was a born leader and is an experienced alternative to David James in the City goal.