2001 Vliegen turned professional in 2001.
2006 Vliegen was a semifinalist in
Chennai in 2006 and in May of the same year, in May of the same year, he reached the final of the ATP tournament in
Munich, setting up the first all-Belgian men's singles final against
Olivier Rochus. He was also the 30th seed at the
2006 Wimbledon Championships, where he reached the 2nd round before losing to
Nicolas Mahut in straight sets.
2009 In Doha, the first tournament of the year he defeated Spaniard
Óscar Hernández with 6–1, 6–7 and 6–7. In the next round he faced German
Philipp Kohlschreiber. He was defeated in three sets 4–6, 7–6 and 6–4. At the Australian open he met Italian
Simone Bolelli but lost in three long sets 6–7, 5–7 and 6–7. One week later he started in the
SA Tennis Open as the seventh seed. In the first round he won in two straight sets of unranked
Ross Hutchins. In the next round he defeated Czech
Jan Minář. In the quarterfinals he lost to world number 6
Jo-Wilfried Tsonga in straight sets 4–6 and 1–6. At the
Open 13 in Marseille he faced Czech
Jan Hernych in the first round but lost in three sets: 6–3, 3–6 and 6–4. ==ATP career finals==