Right.....well lets consider a headgasket setup ala this:
Which is a V6 head.
Blue - Water cooling jacket - will be at pressure
Brown - Oil return to block - kept at a slight avccum thanks to the engine breather system setup
Red - Oil feed - High pressure.
So, under a head gasket failure we can get a few scenarios.
A) Failed fire rings. This results in exhaust gases in the coolant and major pressurisation plus coolant being pulled into the bore on the inlet stroke.. When turned off, the cylinder pressure drops to atmospheric water (which remain at pressure for some time) will enter the bore. Classic symptoms are miss fires when starting.
B) Failure of the coolant jacket. This results in either a coolant leak external to the block (seen on some V6 engines) or water in the oil (very unusual on the V6 due to the small aand confined areas of the oil returns)
C) Failure of the gasket around the high pressure oil feed. This can result ina ajor external oil leak (rare) or possibly oil in the water (also very rare). Thsi failure is unheard of on the V6!
Your symptoms are coolant loss and a non working heater.
The latter we already know is due to a vaccum issue somewhere.
The coolant loss is the one that needs tracking down....it could be a number of things.