It is possible to use Chromecast with FREEDOME VPN on your devices.
When you use Chromecast with FREEDOME VPN, you must use screen mirroring. If you’re casting the content, what actually happens is that your device sends the link to the content to your Chromecast device. Then, the Chromecast device downloads the content from that link. In this way, it totally bypasses your device with FREEDOME VPN.