What is the vent on bottom of reservoir tank?

This is the overflow for the reservoir. If you overfill the reservoir the excess will drain out. If it's not overfilled and the engine is boiling over when you shut it off, that's not normal. To check if coolant is flowing through the heater core, carefully feel both heater hoses. They should both be hot. If they are you have a problem with the temperature door under the dash. If both hoses are cold or cool you probably have a coolant circulation problem. Possibly from a bad water pump. If the inlet hose is hot but the outlet hose is cold or cool the heater core is clogged. Worst case scenario is this is an early symptom of a blown head gasket. Hope that helps! Jim