VyOS is using quite unusual filesystem layout.
The system is located on a single partition.
The system partition contains the following directories (for an image named "image"):
/boot /grub /image image.squashfs live-rw/ <kernel, initrd, and other typical /boot contents>
Every image gets its own directory, named the same to image name. When the system boots, squashfs image and live-rw are mounted in a union filesystem.