There is a vacuum reservoir in the area you described. It sits beside the brake booster on the firewall and looks kinda like a notched box. On my car, the vac connection is on the bottom of this plastic reservoir and is kind of hard to see. This is the vacuum reserve for your air ducts in the heater system. If the ducts aren't working properly, you will know that is the missing line. The small line going to the LF of the car feeds the cruise control vacuum motor.
Just to the left of the booster in the picture below: