Hey, usually it should be possible to change the boot device by pressing "esc" directly after starting the NitroWall (and a connected display). We had some reports that not all usb-keyboards work without issues (slow key-press recognition), maybe using another keyboard might already help. For you VyOS issue: generally we ship OPNsense exclusively as a pre-installed solution as this is the solution we regularly test, please understand that it is not possible for us to test a wide range of network/firewall-systems. That being said, I did some searches and the VyOS hardware support page is lacking the Intel Celeron J-family processor in any of the recommended systems, what might be an indication that the CPU is missing some instruction. Sorry that I cannot help more here...