Hi,
I do run VyOS on a fitlet2 as a (backup) travel router and installation was with serial only (no monitor while on the road). The challenge was that the serial port is on ttyS2 (and ttyS6).
Changes for booting the ISO are pretty standard. Edit the ‘Serial console’ grub entry (press e) replace the console=ttyS0,115200 part of the serial boot entry with console=ttyS2,115200 and boot by either press Ctrl-x or F10.
The trickier part was making the change permanent which involved finding out which of the many grub config files to modify. I ended up modifying
/boot/grub/grub.cfg.d/25-vyos-common-autoload.cfg
Before the line function setup_serial I did insert
# set my serial to ttyS2
console_num=2
… that worked. And in my case it did even survive an image update.