It is called condensation.
That "mist" is more than likely coolant steam coming from a leaky heater core. Replacing the heater core is your best option.
replace? why not fix it?
My ac blows fine through the overhead, floor and side vents but intermittentely throught the front vents. What might be the cause?
Nope, that would be steam.
The musty smell coming from your AC vents could be caused by mold or mildew growth in the system. This can occur due to moisture buildup in the ducts or filters. Regular cleaning and maintenance of your AC system can help prevent this issue.
assuming this is Home AC question - speed up the AC fan speed
I don't think it is coolant. It is just condensation caused from high humidity and the cold air. If it is coolant it should get on the floor and you should loose coolant.
is it because of a lack of vacuum pressure?
Perhaps a dead mouse.
The "fumes" are more than likely antifreeze from a leaky heater core.
There may be something blocking the deflectors from sending air up there.
If antifreeze is filtering through your ac vents, other toxins could also be coming through. Don't take chances; see a mechanic.