Concerning the round pulling a vacuum and sucking the combatants out of their hidding places, IMO I think, as another poster mentioned, is not caused by the round.
The muzzle blast of the gun is an explosion much like any explosion like a bomb (without the shrapnel). It would be so great that I have no doubt the concussive force would be enough to kill when positioned close to it in front of the muzzle. And, like an explosive, there will be an instant increase in air pressure. That ball of increased pressure would probably create a vacuum as it passed forward but it would be behind the blast, which would have seriously injured or killed anyone nearby.
A little farther down past the direct effects of the blast, the shockwave from the 120 mm would be enough to knock you around. If someone were next to a rigid surface behind cover but far enough away from the direct blast, the shockwave would probably knock him against the surface and he would bounce back out from the cover. The surface probably would also have some vibrational movement from the wave also to help propel the guy away from cover.
Even farther down where the shockwave loses force, you have the effects of the round. IMO, the crosssection of the round and the sonic wave it produces is too small to pull anyone out of their hiding place. As an illustration, suppose you stand next to the curb and a bus comes by. The crosssection of the bus is so great the the air it displaces first pushes you away then the slight vancuum as the bus passes pulls you back in. Now if a motorcyclist were to drive by (well behind the bus after the effects of the air disturbance from the bus have dissipated), the effects are much less.
All of this is, of course, IMO. I wounder how long it will take for the GSC to start saying the XXX caliber is so powerfull it will pull badguys out of there cover when the round goes by him by simple vacuum.