Not sure blocking vents for any appliance that is in use is a good idea.
Part of the cooking process is reducing the moisture content of the food and if you are like me, I cook by smell more than anything. Most fire and smoke alarms works on similar principles they look for or smell the smoke when there is fire.
I agree with Frank as usual, you dont need a big honking oven to toast break. You can get a small toaster or toaster oven at walgreens for 20-40 bucks. For just warming up cold stuff I just use the microware. No, dont even think of blocking the vents to a microwave, there stuff in there that needs to be vented.
I put the item in as soon as I turn the oven on vs wait for it to warm up. I also cut the oven off early and let the residue heat finish cooking the food.