The vga statement controls the resolution and color depth of your framebuffer screen for vesafb. As stated in /usr/src/linux/Documentation/fb/vesafb.txt (which gets installed when you install a kernel source package), you need to pass the VESA number corresponding to the requested resolution and color depth to it.
Vous trouverez plus de détails à ce sujet dans le fichier /usr/src/linux/Documentation/fb/vesafb.txt.