That's just it... mode 1+3 (VGA) is the OLD method of doing this mod, mode 1+3 is not compatible with any current VGA mods. You need to change your mode, or your VGA will not work.
You need to change it to 2+3 (HDTV) in order to use a VGA BIOS.
After you do that, you can either try a BFM VGA BIOS to test your VGA without flashing, or you can patch your current BIOS and reflash your chip.