You don't have to worry about this too much; all but the oldest VGA monitors typically have very robust input conditioning on the RGB inputs to clamp input voltages that are out of range. They also AC-couple the input so the DC offset doesn't matter.
Therefore it is sufficient to just use a voltage divider circuit; a potentiometer will do. On my JAMMA to VGA adapter I use three 500 Ω potentiometers. You won't be able to damage anything if the potentiometer isn't set correctly: the display will just be too dark or too bright.
To calibrate them, enter the game's diagnostic mode and view the color bar screen. Almost all games can display this. Then adjust the potentiometers for red, green, and blue until the color bars are not saturated and each gradient is visually distinguishable. Now you are good to go.
I've tested this circuit with many arcade boards from different manufacturers and it works fine. As you mentioned, there's a lot of variation across games so it can be worth it to use potentiometers that are easy to turn (the kind with big knurled knobs for example), since you'll need to twiddle them on a game-by-game basis. It won't always be a "set it and forget it" solution.