Do you see a problem if the difference between VDD, VBoot and VIN is Zero?
The answer is yes I see a problem. Specifically with VBoot. However, Vdd and Vin can be tied together and use a common 12V supply. The rules: Vin = Main Input supply. Connect to the drain of the high side switch. Voltage is independent Vdd when Vdd = 12V. We do not recommend you apply Vin before Vdd for any sustained period of time; both going up at the same time is fine. Vdd = 12V supply for driver itself. All operating bias current is consumed from this pin. It is recommended that Vdd powers up at the same time or before Vin. Newer generations of EPC GaN solutions eliminates the potential issue between Vin and Vdd; VBoot is generated by the driver. Do not attempt to supply your own power source. There is active regulation of VBoot internally addressing specific operating conditions during switching to protect the high side gate drive voltage.