Got a 99 Lesabre with ECC. Start the vehicle and the temp display on the control head blinks for a few minutes then stays on steady. Did notice that no air comes out the dash vents, will come out the defrost when set to dash vents.

The blinking ECC display typically indicates that an issue has been detected by the ECC, and a code set.

A brief google discovered this thread on another board:
Automatic Climate Control - blink codes
This may be worth a crack to try and get some answers. Have you noticed any other issues with the climate controls?

Also, when you attempt this, make sure you are able to record the codes when you do it, as it was suggested that doing this causes the ECC to dump the stored codes.

Further to what rjolly posted, when the HVAC control head detects an error it will go into default mode, which is air coming out the defrost only. The blinking light and air coming out the defrost only tells me the HVAC control head likely needs to be replaced. You can usually find them on Ebay for not a lot of money, and they are relatively easy to change.

I tried getting a code out by pressing and holding the off, auto and recirc buttons but I don't get anything.

Sure would be nice to have another panel to try in there. Check electrical connections.
Otherwise a TechII scanner or Snap On Ultra, can read the fault codes, in the hands of a familiar tech.

More likely, a leaking vac tank, disconnected vac line, or bad electronic vac valve box.....lose any of these and the system defaults to the defrost mode....