I'd have to visually look, but on throttle body, there may be two ports...one for emission, and the other for IGN-MOD. When engine is at base idle, there should not be a vacuum on IGN-MOD's port on throttle body.
IIRC, from years ago, on distributor based engines, when doing timing, pulling vacuum line off was a prudent measure...so timing was set with no vacuum going to distributor (or IGN-MOD).
I've not examined vacuum ports on throttle body, but I assume the emission port gets vacuum when engine starts up. It's function is to suck gas fumes from charcoal canister.
Hence, each vacuum port may "suck" differently.