When you use a VGA box with your Dreamcast to get the best possible picture, be sure to patch in support for it in your games.
The best way to do this is get a free hex editor.
Install it and open your .cdi image file.
Search for “JUE” and next to it you see some numbers.
These are winCE settings, peripherals/controllers support but also vga support.
So if you got a game that got that string: 0799A00
Change the BOLD byte to 1: 0799A00 -> 0799A10
And vga is working, don’t change anything else!