Do you mean the ALDA is off the IP or the ALDA is on the IP but not functioning?

ALDAs aren't subject to vacuum but to pressure. You might in fact damage an ALDA by applying a vacuum to determine if it's intact. Instead, apply pressure and look for nothing more than a slow leak. Any ALDA from '78 through '95 should work on your car so there's a wide range of seconds for you.

I'm not familiar with the '85 TD but I don't think the firewall switchover valve has anything to do with the transmission. Could it be you see a line from the switchover valve splitting with one branch going to the ALDA and the other to the transmission vacuum amplifier? From '86 there's a temperature switch and switchover valve attached to the transmission vacuum amplifier. Maybe '85s have it as well.

Sixto
87 300D