As I understand it there are two things you should make sure of.

First the mirror needs to be very clean, especially the surface.

Second the pressure needs to stay low. It is possible for the mirror to give off gasses while it is in the coating chamber. I suggest that you leave the pump on for a while after it reaches the correct pressure in case the pressure does not remain steady. The bigger coating companies use large capacity pumps to make sure the pressure stays low.

Since you have the equipment, it will be inexpensive to try aluminizing a mirror or two. You can strip the aluminum off with muratic acid mixed with a little copper sulfate.

Good luck, and be careful.