The "fumes" are more than likely antifreeze from a leaky heater core.
It is called condensation.
replace? why not fix it?
yup, sign of a bad heater core
Nope, that would be steam.
Generally, with that smell- the heater core has gone bad.
assuming this is Home AC question - speed up the AC fan speed
is it because of a lack of vacuum pressure?
There may be something blocking the deflectors from sending air up there.
Perhaps a dead mouse.
A pipe that vents fumes
Those "fumes" are more than likely coolant steam coming from a leaky heater core. Replacing the heater core is your best option.
If antifreeze is filtering through your ac vents, other toxins could also be coming through. Don't take chances; see a mechanic.