In past threads the devs have stated that they would rather improve the disable toggle than allow WiFi Calling to route through VPN. If I had to guess why, it would be because the software architecture makes it too hard to route system-level traffic through a VPN. I could be totally wrong about this though.
I too would prefer to see WiFi Calling go through VPN, but I would also be satisfied with having a way to completely disable all its traffic. Wifi Calling is practically the only thing on GrapheneOS that still leaks identifying information over the network.