Is it safe to assume that all pins in a standard VGA cable are wired?
Don't assume.  Most VGA cables use some sort of coax or individually shielded conductors for the Red, Green, Blue, and sync signals.  The shields for these signals, which have their own pins, are commonly tied together inside the cable.  Additionally, some older VGA cables might not connect the "unused" pins-- which are used on more modern devices.