If the phone “isn’t doing anything”, my first instinct would be to have it manage the VPN with its otherwise “quiet” CPU. You’d want to double check that the packets are going though your VPN.
Personally, I’d only use the official WireGuard (or OpenVPN) clients on the phone, not some “third-party” or provider-specific app.
if you mean vpn client, yes add it to the slate, if you mean vpn server, i dont think it can unless you find a way for the phone to forward the listen port to the router