I recently inherited an older PC running win7 and I wanted to see if it still works, the issue is I don't have an VGA monitor and VGA is all the computer supports, so at the moment, there is no way for me to see what is going on at the moment, so I was wondering if there is any software available that I can use on my android phone as a monitor via USB where I do not need to install any additional software on the older device. If anyone has any suggestions as to how I might be able to view the device's output, that would be brilliant. Thanks in advance.
It's VGA, not RGB - you'll need some flavour of software for remote access. You can't just plug a phone into a PC over USB and get it to do anything, let alone be a monitor by itself.
Now the solution you need is something that converts VGA to a HDMI or DP - these are commodity items with a bunch of options. You can then plug it into a modern monitor. Presumably you can then set up some remote access software to run it over wifi.