Where should I connect the VCC pin of the HW-USBN-2B cable if I'm programming an iCE40 device?
The VCC pin of the HW-USBN-2B programmer is an input probe. It detects what voltage is being used by your design. You may connect the VCC probe to the voltage used by Bank1 of your design (which is associated with SPI_SI, SPI_SO, SPI_SCK and SPI_SS pins.