are they that vulnerable?
Depends, where the "toy" hits the heli.
I would guess (and only guess):
Through the front pane: Incapacitating the pilot, loss of control -> down
Into the main rotor disc: Damaging a blade, bringing rotor out of balance and destroying aerodynamics -> maybe an autorotation would work
Into tail rotor: same as a hit into the main rotor?
Into air inlet for turbines: Maybe nothing would happen (especially, if filters are installed), maybe the worst thing could result in engine failure, if at least some parts of the drone make it through all the way
I wouldn't want to be the first to find out...
Thracian