- Bats Right, Throws Right
He debuted in 2006 for the Mortsel Stars, going 5 for 22 with four walks and a double. He was 6 for 19 with five walks and five runs in 2007. In '08, he was 4 for 13 with a walk and two dingers. In 2009, still a teenager, he batted .268/.322/.402 with a league-leading five triples. With the 2010 Stars, he batted .295/.391/.358.
Vandesompele stole 22 bases without being caught in 2011, hitting .317/.432/.372 with 41 runs and 29 walks. He tied Steven De Lannoy for the league lead in runs, was 5th in walks (between De Lannoy and Jamie Verheyleweghen), tied Lorenzo Oeyen for 8th in times plunked (6) and was third in swipes.  In 2012, he slumped to .185/.339/.217 with five steals, though he did draw 22 walks in 26 games. During 2013, he hit .257/.341/.301.
Sebastiaan produced at a .406/.454/.566 clip with 37 runs and 19 steals (not caught) in 26 games. He was 4th in average, 6th in steals and 4th in runs. He made Belgium's team for the 2014 European Championship, backing up Dennis Ribbens in left field. In his debut, he batted for Ribbens with the bases loaded and a 10-0 lead against Sweden. He hit a sacrifice fly off Kent Karlsson to score Dennis De Quint. He started in left and batted 7th against France. He flew out twice against Owen Ozanich, drew a walk from Jonathan Mottay and scored and reached on a fielder's choice versus Pierrick Lemestre and scored again in a 5-4 loss. Finally, he replaced Vincent King at DH in a 12-2 win over Russia, being retired by Denis Leonov twice. 
He was at .323/.426/.435 with 26 runs, 22 walks and 20 steals (in 23 tries) in 2015. He was 5th in stolen bases. In '16, he batted .215 with 14 swipes. The next year, he hit .283/.421/.391 followed by .283/.421/.391 in 2018. His 2019 batting line was .308/.370/.396 and he was 6 for 14 in 2020, shortened significantly by the COVID-19 pandemic.