Please refer to section 5.3 of the 4iiii iOS App User Guide below for more information on configuring the ANT+ to BLE Passthrough with Viiiiiva.
The most important step is to tap 'Update' at the bottom of the screen to complete the configuration.
Then to connect the Viiiiva, and ANT+ sensors to the desired BLE only device or app, you will be connecting 'Viiiiiva' as both HR and the desired sensor (ie. connect 'Viiiiiva' as bike power). You must first delete the Viiiiiva from the 4iiii App before connecting it to another BLE device. Viiiiiva can only be connected to one device or app at a time via BLE.
Some training applications require the Viiiiiva to be named the default name 'Viiiiiva'. If you have previously renamed the Viiiiiva to a custom name, please rename it back to 'Viiiiiva' prior to connecting to your desired training app.