Dreamcast Guides : Manually Enable VGA

-open the file in the hex editor (preferably CDI format)
-search for the string "KATANA"
-shortly after, you should see "JUE" followed by some numbers
-change the number string after "JUE" to "0799A10"
-save the file ( I recommend over-writing the original file, so you don't accidentally burn your old, non-patched image)

NOTE: Some games even after being patched react weird, Bangai-o will only display in the upper corner of the screen while some games like PAL version of Skies of Arcadia will never work patched. But keep in mind the Dreamcast is region free and some games like Skies of Arcadia NTSC-U DO have VGA enabled.