Is part of it a Grey hard plastic hose?
If so it goes to the Heater hose By-pass valve, has 4 heater hoses connected to it, yes a vacuum hose
End often broke off so rubber hose was used as a "patch"
Other end runs thru firewall and goes into the cab under the glove box next to passenger side kick panel
It changes to a White hose in the cab
There will be a black hose running next to it, this is a main vacuum hose from vacuum reservoir in the engine bay, it runs the cab vents, defrost, panel, floor
The white/grey hose only has vacuum in OFF and MAX AC settings, so not "a vacuum leak" as far as engine or cab vent system is concerned