This confirms nothing. The vent pipe WILL suck in any installation. It only matters if the pressure difference between the cylinder and vent pipe outlet is enough to suck so far that the water column entirely disappears so that air enters the system. I see nothing to suggest that this has happened. The symptom you are looking for isn't reduced pressure and a bit of sucking at the vent (which will happen in any system, including a healthy one), but funny noises and entrained air in the hot water stream.
Christian.