Just a bit of food for thought.
The vortex will be affected by the wind on the day. So given a 15 kt headwind, the vortex will drift around 500m down the runway from the point at which the 75 had rotated (in 1 min - back of a fag packet calculation). Not insignificant, and more to the point, probably at the point the RJ will rotate. Add to this the fact a 10 knot crosswind will hold the vortex of the upwind wing over the runway. So a NW/NE/SE/SW wind at around 15kt would not be too good for the RJ boys in this situation.
Second bit of food (must be a nibble) is that it seems to me that the RJ climbs out a lot shallower than the 75 (must be something to do with having too many engines). So say the RJ rotates before any vortex, it will then has to fly through it straight after take off. Bumpy rides for the RJ boys....
SS - I hate it when I get stuffed by the slot situation too. Life...