if you have a stock air cleaner, it would go to the vacuum switch mount in the base of the air cleaner. the other side of the vacuum switch goes to the vacuum actuator on the aircleaner snout to divert the carb air from the snout to the heated air intake manifold mounted on the exhaust manifold.

if you dont have that setup, just cap it off. a vacuum leak will hurt your vacuum signal to the carb and hurt your performance (youll run lean)