Any ideas how should we start the CLI?
Basically, two options.
First, vyatta style: upon login the user is in the VyOS-specific CLI, where you can do both VyOS and UNIX things.
Second, juniper style: the user is dropped to UNIX shell and can do UNIX things right away or start VyOS-specific shell and do VyOS things there.
Second option is a lot easier to implement, but some people may get confused and try editing configs in /etc/ by hand, with confusing consequences. They already do it in our current CLI once in a while. We can probably mitigate it by making the post-login message tell people what to do.