2000 buick lesabre blows A/C air out of defrost vents only. No air coming out of dash vents.

Look inside the glove compartment for the vent control they are probably run by vacuum. See if they are leaking.

blend door stuck or actuator failed

Sounds like vacuum switch is either bad or there's a vac leak in the duct controls

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

MODE DOOR NOT SWITCHING FROM AIR TO HEAT IT'S UNDER THE DASH THERE ARE TWO 1 THAT WORKS THE VENTS ONE THAT WORKS THE DOOR FOR THE HEAT AN AIR IF VENTS CHANGE FROM DEFROST TO DASH AN TO FLOOR IT IS THE MODE DOOR actuator

underdash near glove compartment a solenoid pack with plastic tube different color you can find your problem

Usually a blend door problem.
Doors under the dash direct air thru the system.

The actuator door for the middle ducts is not opening.

They have two types of air conditiong controls. AUTOMATIC & MANUAL . Your mode switch maybe bad if manual. If automatic the head is probably bad and they are sold as exchanged units only. If manual check all circuits to see if 12 volts has continuity

check for vac leaks under hood and dashbord