No, when a Vilo is powered off, the firmware will not be upgraded on the Vilo. A Vilo needs to be powered on in order for the firmware to upgrade.
In the scenario where one sub-Vilo is powered off, the firmware on the rest of the Vilos in the Wi-Fi network will be upgraded. The firmware of the powered-off Vilo will remain unchanged. If you want to upgrade this Vilo’s firmware, power it on, and add it back to the Wi-Fi network, you will be prompted to upgrade the firmware of this sub-Vilo or you can go directly to settings and upgrade.