If I turn on the HVAC to any position where it's suppose to vent from the dashboard towards the cabin (VENT, NORM A/C or MAX A/C - manual controls), it vents in defrost mode to the windshield instead. After some time goes by, and the engine is at idle with high vacuum (e.g. at a stop sign), it'll switch to the dashboard air vents on its own.

If this was a constant vacuum leak, I reckon it would continually be stuck in defrost mode, but this one has me scratching my head. Pretty sure it's a vacuum issue, considering my auto parking brake release also stopped working. Thanks!

Definitely a vacuum issue. Maybe the vacuum accumulator or the line going to it? There's only 2 miles of vacuum lines in these cars, so have at it!

Do you have manual or electronic controls?
The EATC unit uses o-rings which eventually crack and leak. I would suspect the one for the panel door has cracked. #007 is the size of the o-rings.

It's also possible the panel door hinge has broken, leaving the door stuck.

Given the frequency of o-ring failure, that'd be my first bet, fortunately its cheap and easy to replace them all. Everybody who buys them has extras left over.

Definitely O rings.

What's the likely issue if the airflow is always at least slightly coming from the vents? It'll still switch between the other modes but it's always blowing a bit from the vents, it's easy enough to close them but I'm losing plenty of airflow and heat that way.