I believe the below should apply to the 510, 520, and 540. I only have access to a 540 and have only tested there.
These machines have a CP2102 USB UART interface built into them, with a miniUSB port on the side for accessing it. This port is powered by the USB host it is connected to, which is nice because that means it remains active even if power is removed from the velo board.
By default it runs at 115200 8n1. It is not ttyS0 internally, but ttyS1. I found this by looking at the grub config of the openwrt image from modlog:
serial --unit=1 --speed=115200 --word=8 --parity=no --stop=1
terminal_input serial; terminal_output serial
set default="0"
set timeout="2"
set root='(hd0,gpt2)'
set force510="velocloud.board=edge510"
set force520="velocloud.board=edge520"
set force540="velocloud.board=edge540"
set force520b="velocloud.board=edge520b"
set force540b="velocloud.board=edge540b"
menuentry "Velocloud Installer (edge5x0) DMI based" {
linux /boot/vmlinuz root=PARTUUID=D76572FA-5473-4A28-B943-6C03504CCF0D rootfstype=ext4 rootwait earlyprintk=serial,ttyS1,115200 panic_timeout=5 console=ttyS1,115200n8 noinitrd vc.install=83890d69-8427-48f3-9e58-cbd28feda5e8
}
menuentry "Velocloud Live" {
linux /boot/vmlinuz root=PARTUUID=D76572FA-5473-4A28-B943-6C03504CCF0D rootfstype=ext4 rootwait earlyprintk=serial,ttyS1,115200 panic_timeout=5 console=ttyS1,115200n8 noinitrd
}
menuentry "Velocloud (failsafe)" {
linux /boot/vmlinuz failsafe=true root=PARTUUID=D76572FA-5473-4A28-B943-6C03504CCF0D rootfstype=ext4 rootwait earlyprintk=serial,ttyS1,115200 panic_timeout=5 console=ttyS1,115200n8 noinitrd
}