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Hi and thank you for your question.
It is not a vacuum line. It is a vent line for the tank. You should also have another line coming off the tray underneath the fuel level mechanism as well.
Here is a diagram of the tank and lines:
As long as you dont have a California compliant bike, the vent just routes down past the engine and stops.
Were you able to identify the hose in the parts diagram? CA. bikes route all vent lines to the charcoal cannister. That however should not affect running. Could you post a picture of the mystery line?
The routing of the vent lines are different, so yes. This sounds like a vacuum line however, so it is either coming from or going to the intake manifold.