This is an unusual use case. Remember that VT1350 is only for 1080p (or GPU scaled resolutions based on 1080p). For 640x480 it is usually easiest to just use GPU scaling for this, and use the VT1350 trick, but that often blows it up to the whole screen surface instead of 17 inches or 1:1 pixel mapping (tiny ~12 inch image)
You could just skip GPU scaling and just send a 640x480 signal to the monitor, and let the BENQ monitor handle the modes. BENQ's menus have support for windowboxed modes, but it might not be able to do 640x480 at the exact size you want... I have no idea what VT is possible with 480p but it might be VT600 or VT800, etc. Remember, VT is Vertical Total based on visible vertical resolution plus the amount of hidden blanking interval (VSYNC) lines/pixels.Little-known CRU windowboxing trick
There's something I rarely do (and few do) to bypass problems. To solve the problem is somewhat much tricker, you may need to windowbox your 1080p by transferring Active to either Front/Back Porch. For every
2 pixels removed from Active, add 1 each to Back Porch and Front Porch. You're reducing your resolution while transferring to the porch values. This thickens the black bars around your image while keeping things 1:1 mapped on many displays. Repeat separately for the horizontal and vertical dimensions. Keep dotclock and Totals (H/V) exactly the same. Sometimes the resolution windowboxing trick works perfect, and sometimes it does not. Windowboxing via CRU (adding surrounding black bars around a lower resolution) requires 1:1 pixel mapping, but it may be possible to combine GPU scaling with it (I have not tried). Basically, you windowbox down to say, 1024x768 1:1 mapped in center of the display via a custom CRU resolution. THEN use GPU scaling to scale 640x480 (into the windowboxed "1024x768-in-1080p" CRU mode). What happens is that you have a lower resolution embedded in the middle of a higher-resolution 1080p-like signal -- and sometimes it works on the monitor. This can work kind of weird sometimes, especially if the monitor is insisting it's a 1920x1080 monitor (over EDID) instead of 1024x768. I have not tested this on BENQ monitors, and CRU windowboxing tricks are rarely used as they are confusing -- but it works on some displays. Windowboxing all the way down to 640x480 1:1 pixel-mapped is possible, but may cause some displays to go out-of-sync, or do weird stretching of the image. (Tweaking monitor menus, such as "1:1 mapping", can help a lot in prevent auto-sizing images).CRU glossary
To better understand the purpose of VT1350, check Custon Resolution Utility 101
for what the heck a "VT" is and why "1350" is used...NOTE: Normally "windowboxing" is a common term used when you see a TV broadcast with black bars all around (top, bottom, left, right), but this little known CRU trick (transferring Active to Porches in exact dotclock-maintaining transfers) allows you to windowbox a lot of resolutions in an unmodified 1080p signal, also great for sending non-GPU-scaled signals to HDTV televisions that doesn't support non-HDTV resolutions -- e.g. no support for 640x480 or 1024x768 is needed in order to successfully windowbox 1024x768-inside-1920x1080p signal.