As kaotikzen said, disable all the fb drivers. You only need KMS. Besides that, you will need some splash tool, like bootsplash, splashutils, plymouth or the like. This suggestion is only aimed at giving you a hint on what to research, since I don't use bootsplashes myself, and I know not the current status of this (in Gentoo or any other distro at all).
If all you want is the resolution but without any bells or whistles, then KMS should be all you need. Just make sure that no other fb thing is getting in the middle. And make sure you compile all the kms related drivers as 'y' and not 'm', so they can get into scene as soon as the kernel is loaded.
You should also research on grub2, and how to configure it properly for vesa modes, since until the kernel is loaded, grub is everything that reigns over your hardware, including the video output. Part of the story is setting in grub the same resolution you will be using in kms once the kernel loads.