I finally figured out the vacuum hose thing. I just needed to poke around some more to find that nipple.
MIDDLE UNDER BELLY TRAY
Since the Geo is my beater car now, I simply drilled straight into the chassis.
I cut up the tray into smaller pieces so that the mounting points will have less weight to bear, hopefully lessen the chances of pieces falling off. This method has worked well so far for my Sti. This is the longest my Sti had gone without the under tray falling off.
Next up: front and rear under tray, lower grill block.