I have a 1999 Buick LeSabre. My AC blows cold air out my windshield and floor vents but not out of my mid or panel vents.

There is a vacuum hose that is no longer connected to the engine intake manifold.

Cornelius, vent problem could be a pinched or loose vacuum line on controller,faulty AC vacuum motor, faulty AC programmer,etc. needs to be checked out by a good Tech, diagnosed and estimate on repair.
As far as vehicle only blowing cool air, issue could be blend door issue, plugged AC condenser core dirt/debris, low AC refrigerant, plugged AC lines internal, failing compressor, etc., again should be checked out, diagnosed and estimate for repair by a good Tech.
Check the attached links,instruction and guides, Good luck
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1998 Buick Lesabre No Air From Dash Vents
http://repairpal.com/ac-vent-problem-346
Car AC Not Blowing Cold Enough Car Repair Information From MasterTechMark
My Car Is Not Cold What Could be Wrong Elite Auto Repair

There are a number of things that could be going on. Do you get cold air out of the passenger side vents? Have you tried sending the air to the defrost vents, or the floor? Do you get cold air out of those when you do?

your heater core is possibly stopped up if there has been a radiator leak and stop leak has been poured its definetly clogged or could be a faulty thermostat

The actuator door for the middle ducts is not opening.

Inside the heater box (under the dashboard) there are actuators that are similar to the ones that open your power door locks, they do go bad especially if any water leaks in from a bad heater core or windshield leak.(down the defrost vent grate). Also old age, my truck is a 2000 and I have replaced a couple of actuators.

The vents are electronic controlled actuators not vacuum.

if car has had dash work vent tubes may not be attached to vent opening if the job wasnt done properly-or tubes may be cracked or missing

it sounds like a vacuume motor that is out or going out also check all vacuume lines for cracks

check for vac leaks under hood and dashbord