There is a vacuum reserve canister mounted under the battery tray that is required to run the vacuum accessories when there is low vacuum at higher rpm's.
thank you- also interesting... it looks like a factory plug on that line. but a reservoir would make sense to have on that circuit. i will have to revisit that later
Trace that line back to it's source and IF it's connected to a T where one line goes into the firewall for the A/C controls and the other line goes to the intake manifold source; then the plugged end does in fact go to the vacuum reservoir that is bolted to the lower section of the fender well near the firewall.