Hi there
are you constrained by a company paying school fees for only these 3 schools? The reason I ask is that there are plenty of more schools in English in Brussels / Waterloo than these 3. Off the top of my head there is : BISB, BJAB, BEPS, WIS, BICS, Acacia, St Paul's, European Montessori, International Montessori Woluwe, AC Montessori Kids, all of which all go up to at least 11, other schools only go up to 6, so that is 10 other schools you could choose from.
BSB : in Dutch area and commuting to Vilvoorde means either sitting in a traffic jam on the ring or using the non ring route which will still be busy, French areas not far but not walking distance
ISB : in French area, even further to commute to Vilvoorde
St John's : a really long commute to Vilvoorde by Belgian standards
BISB, BJAB and International Montessori Woluwe are the best situated schools for Vilvoorde, the former 2 are schools right in the city, the Montessori is in a suburb officially Dutch but in reality about 50/50 French/Dutch.
I would enquire about whether you could get a full-time place for the 3 year old at any of the 3 "big" schools, you may be too late. For the baby, only BSB has provision for 1 to 3 year olds, but places in that age group are very very popular.
Would your husband's commute to Vilvoorde be possible by train or bus? Or by bike even? I would take public transport links into consideration. If you want to live in a French area and work in a Dutch one, you often find the French / Dutch politics means that connections are not always the best!!! I live in French majority east Brussels, but the commute by public transport to Dutch Vilvoorde is not easy.
Finally is English an essential part of education? The local schools are often great.