Which is a more effective cooling method? Assume I have a refrigerator at 0C and I put a can of soda at the center of the refrigerator. The temperature inside the refrigerator will be increased to 5C after I put the soda in, but will go back to 0C after a while
If I consider the speed of cooling,
should I keep the pressure inside the refrigerator be atmosphere pressure? OR I should create vacuum inside the refrigerator
If I consider the energy efficiency, 
should I keep the pressure inside the refrigerator be atmosphere pressure? OR I should create vacuum inside the refrigerator
 A: You should keep it at atmospheric pressure because of the thermal conductivity. 
A vacuum is an under pressure or a space that is devoid of matter, therefore it is a very good isolator. The second best isolator is stationary air, so if you want to cool the can as fast as possible you should have a decent airflow in the fridge to abstract the heat from the can.
A: In a vacuum the can would be unable to give any kind of convection current to the fridge so only radiation and conduction would be allowed. since the can is at a similar temperature to the fridge the radiation exchange would be very small. the source is not powerful enough to initiate substantial heat transfer. And if it was on the rack, then only that rack that was in contact with the can would be working on transferring particle intensity.
At atmospheric pressure, you would have an abundance of particles in contact with the can. So convection currents would allow for a 3Dimensional heat transfer.
I would also add that if the fridge was under vacuum the can itself would expand a bit due to a weakened normal force acting on it.  Which would reduce internal energy intensity of fluid inside due to increased volume area. so heat transfer would be even slower.
