I've not connected that monitor to my Xubuntu systems so don't know what it will do there.

Before I discovered the need to connect the VGA cable, the monitor appeared to be totally dead! After connecting it, the display started as 'Analog' and later switched to 'Digital' automatically. That machine's BIOS supported only VGA, while its video card made the switch to DVI during its initialization. The Viewsonic on my wife's older machine (now retired) had to have both the VGA and DVI cables connected, before it would display during the boot sequence. However there could have been other factors, too. There's also a log at /var/log/Xorg.0.log that will detail exactly what happens during initialization of the display.

If you want to see all the gruesome details, you can examine the files /var/log/dmesg and /var/log/syslog to see what happens, step by step, but be warned that it really is a huge mass of detail and quite easy to get lost in.

I suspect that you rebooted and that's what made it work. Linux systems, unlike Windows, customize their configuration to match the hardware each time they power up rather than at install time. As Dennis said, the response from Viewsonic simply means that they don't provide tech support for systems other than Windows and Mac the monitor itself willl work properly with any system that sends it the signals it expects. HID driver for ViewSonic devices not fully compliant with HID standard.

Viewsonic drivers for linux