Vacuum lines are not dangerous. If the line has come off you need to locate it and reconnect.
Start at the engine bay along the firewall. On the pass side the heater core pokes through the firewall. In the same grommet, is a small black vac line. Is it connected to the main lines in the engine bay?
If so, your next stop is pulling the hvac panel out and making sure those vac lines are properly connected.