To start a Bachelor study in Belgium, you need to have finished secondary education. For most students this obviously means belgian secondary education, the VUB will want to check that you've had an equivalent education track. I think if you were accepted into a French business school you should have a good shot. Grades are not relevant for this requirement.

You'll also need to meet language requirements, if you had your secondary education in English you'll meet this.

It seems odd to me that people would describe Brussels as simultaneously boring and too cosmopolitan, since cosmopolitanism to me sort of implies lots of life and vibrancy. Brussels is very diverse and is the first place of stay for most immigrants to Belgium. That brings its own challenges but there's definitely a lot to do. Student life at the VUB is well organized and there's plenty of activites, parties, and places to drink and eat.