The arm64 kernel config (vyos-build/packages/linux-kernel/arch/arm64/configs/vyos_defconfig) has not been updated for kernel 5.15.
In the course of updating the VyOS build on our hardware (Ten64) I have updated it.
This has been mostly lifted from the Traverse config over at https://gitlab.com/traversetech/traverse-kernel-patches/-/tree/lts-5-15 .
I cut and pasted the 'Networking Options' block from the VyOS x86 config so the network stack should be the same.

Note that a lot of drivers for graphical, multimedia and other devices unlikely to be used on an [Arm] router have been turned off to save compile time (especially as I frequently recompile for testing and verification). At some point once we are more confident we can adopt the kernel or Debian defconfig which turns on everything.

This kernel will work under QEMU as well and should work on a few other machines (e.g VMware, AWS Graviton) but I have not tested widely yet. Raspberry Pi 3/4 _should_ work as well with an EFI capable bootloader.

(Pull request incoming)