Are you saying that the vaporized matter ejected from the blast will not contribute to moving the main mass in the opposite direction?
Or are you saying that the entire asteroid, or enough of the asteroid so as to leave only insignificant bits behind, will be vaporized during the blast(s)?
As for the first, the vaporized matter certainly does propel the asteroid. If you take some gravel on the asteroid and make it go thataway then the rest of the asteroid will be pushed an equal amount in the opposite direction. Now vaporize the gravel and make it go thataway... the asteroid's still pushed an equal amount in the opposite direction.
As for the second, breaking up a asteroid into chunks, say, less than 10 cubic meters would certainly keep it from being a single large impact on the Earth.
Take a 1 km chunk of rock and hit it with a 50 mt blast and there probably won't be much left of a solid core. But the whole thing probably won't be vaporized, either. You've created millions of small, fast, randomly distributed moving masses. Better, IMO, to calculate the blasts appropriately so as to leave the mass of the asteroid intact.