A simplistic VGA to XYZ-scope converter like this:
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just uses a passive resistor network to linearly scale the RGB components and sum them to monochrome.
However I don't think this really works for a "modern" SVGA output because the gamma will be wrong; you have to apply a nonlinear gamma-expanding transform prior to intensity-scaling a la:
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Is there any off-the-shelf IC or circuit that does this or are you basically looking at using a DSP or FPGA to do it properly.