The plugged vents shouldnt affect anything. Same thing happened to me this winter in -35C. Overheated like crazy and had no heat. I figured my coolant had slushed up and was making it overheat or my thermostat was frozen. I was right, i parked in my heated underground parking overnight and it was back to normal the next day. I would say your thermostat is stuck closed for whatever reason frozen or faulty.