Heres how I would do it:

C-Script:
ifndef VID_MODE;
define VID_MODE,8; // Change the 8 to your default mode
endif;

ifndef VID_DEPTH;
define VID_DEPTH,32; // Change the 32 to your default depth
endif;

video_mode = VID_MODE;
video_depth = VID_DEPTH;

The ifndef checks to see if those were defined (from the command line), and if not it defines them to whatever you want the defaults to be.

And start the game like so:

mygame.exe -d VID_DEPTH,8 -d VID_MODE,32

You could also do it other ways, like:

mygame.exe -d VID800X600X32

ifdef VID800X600X32;
video_mode = 8;
video_depth = 32;
endif;

// Repeat the above for all other modes you want.
// If you dont define one, you get the acknex.exe defaults.