Normally a stopover must be at a connection point (or even hub) along a normal legal routing.I suspect that DL doesn't offer a through fare from BRU to GRU with a connection at YVR; it's too far out of the way and for fare rules that use a maximum mileage requirement rather than specific allowed routings, YVR would surely exceed the miles permitted for BRU to GRU. Some fare rules further specify continents on which stopovers can occur.
A general comment is that delta.dumb doesn't do very well on complex and/or multicity itineraries, so it can b useful to call and ask them to put together a ticket with the segments you want; you should do your research and decide what you want before calling.
ADDED: for this itinerary on DL, ATL would be the obvious connection point/stopover. In the USA, JFK and even DTW might be allowed. Some fares could allow AMS/CDG/LHR too, but I suspect you're not so interested in such stopovers. By contrast, if you fly AA, then MIA would be the obvious connection/stopover and others that might be allowed would include MAD (using partner IB), LHR, DFW, ORD, CLT, etc.