> though from a quick search it doesn't seem to have a "Vagrantfile" equivalent (is that correct?)

It's just YAML config for the VM's resources:

https://linuxcontainers.org/incus/docs/main/howto/instances_...

https://linuxcontainers.org/incus/docs/main/explanation/inst...

And cloud-init for provisioning:

https://gitlab.oit.duke.edu/jnt6/incus-config/-/blob/main/co...