Lets guess, the spare vent, is the one in your bedroom ? of is the
vent in your bedroom open to the boiler room somehow ?

The vent in the bedroom has next door's boiler's flue running up it.
This is the right-most vent as you look from our house. The middle one
has our boiler's flue and the left-most one has our woodburner. Our
boiler's flue enters the chimney higher up than next door's, so I
assume next door's flue runs horizontally through the middle vent to
get to the right-most one. I can only assume they've fixed up the
brickwork to keep the vents separated!