Not all machines ship a serial interface anymore.
On my ESXi instance I stumbled upon Nov 29 10:34:41 CR1 agetty[12394]: /dev/ttyS0: not a tty in my systems logs b/c I have not specified a serial console.
Also GRUB doesn't like it
We should make VyOS more intelligent to autodetect the serial console or even better, make it configurable. Could be that someone has a serial port but does not want a console?
Looking through the sources, I found ./data/live-build-config/includes.chroot/opt/vyatta/etc/config.boot.default which is the default configuration that will be installed. It holds a serial interface.
console {
device ttyS0 {
speed 9600
}
}