Solvay or Vlerick Business School

No direct experience, but I do know a graduate MBA from each. I'd say that Solvay was a lot more traditional, and very Wallon, Vlerick is a more modern course, with a lot of up and coming execs mainly from Flanders. Antwerp Management School is another one to consider. Internationally, I believe that Vlerick consistently scores higher than Solvay on the various rankings.

Having done MBA from Vlerick, I can tell you why I made the decision to go for it:
1) better recognized in Europe (especially for Executive MBA)
2) better ranking
3) More mix of students from all over the world. In my class batch(2010-11) were 41 different nationalities. Just having to hear their views during interaction was itself amazing.

However, I should tell you that finding a Job post MBA was challenging but I guess that would go for both Vlerick and Solvay. Langauge(NL/FR or both) is crucial.

Language skills are very important in Belgium.
They will hire preferably someone who cans speaks 3-4 languages iso a more qualified person speaking only one language.

Having done part time MBA from Vlerick, can tell u from my experience career change won't be easy unless you speak at least one local language fluently or if your current company has a career change position reserved for you but MBA is the only thing missing. Vlerick or Solvay MBA does add another star to your CV but it wont have the same value in the local market here compared to the other top international MBA's although the content of the course is at par with the international ones.