I have a question ADUM1250.
Is it possible to turn communication ON / OFF by turning ON / OFF the power supply to VDD 1 with FET?
No, this would not be possible. The SDA1 and SCL1 pins have ESD diodes to VDD1. If power was removed from VDD1, the VDD1 side of the ADuM1250 would be parasitically powered by a high signal on SDA1 or SCL1 pins through the ESD diodes.
The ADuM1250 is designed for hot swap applications. The SDA2 and SCL2 pins do not have the ESD diodes to VDD2. It may be possible to flip the ADuM1250 device around . There are other differences between the primary and secondary side to consider. Please refer to Datasheet and User Guide on the ADuM1250 page here: http://www.analog.com/en/products/interface-isolation/isolation/isolated-i2c-isolators/adum1250.html#product-overview