I rather doubt it's vacuum related. With how much force does it blow out the windshield vents when in defrost mode? No vacuum is required for defrost, that is the default mode for the vent system.

I suggest you learn about and chase down the blower motor and regulator. Both are under the hood. You need to remove the windshield wiper assembly to access the blower, instructions can be found on this site. The blower motor may be binding up due to wear and age, or the regulator may be failing to deliver full voltage to the blower.

- JimY