I had a user complain a font was missing she liked. She told me was called "system". Looked in several windows computers font lists (both 7 and 10), and there is no font called "system".
But in playing around, I noticed in both Word and Libreoffice, if you type in the word system in the font selection box, you get a font called "system". What is this font and where is it really coming from? Both "Microsoft Sans Serif" and "Arial" are very close fonts, but neither is a perfect match for "system". Font identifications sites Fontspring Matcherator and Identifont both failed to ID it.
System isn't a specific font, but rather an old Windows remnant; a shortcut in the font menu options to choosing the computer's set default system font - in this case it would appear your system font is set to (or the font file that vgasys.fon is recognizing) is Arial.
I found the answer. This is font substitution at work and is the work of the program, not the operating system. There is nothing special about the name "system" other than I have no font with that name. I can type in "xyz" as the font name in the font selection box and get the exact same behavior.
You can also see this behavior (which is how I solved this) if you create a document using a font on one system, then open that document on another system lacking that font and if you fail to embed the font in the document.