Why should a Piper Seneca not take off after an engine failure below 100MPH?
I finally found out an answer which makes more sense to me . By test pilot , vmca masured with a lot of citation and one of them Is 5 degrees angle of bank towards the live engine . So it means that we can't have that 5 degrees because we are still on the ground.
for each 1 degree , our speed decreases 3 knots .So 5 degrees = 15 knots which is equalls to 20mph !
 80+20 = 100 mph ! 
This was the best answer I have got until now.