According to http://www.funtoo.org/Funtoo_Linux_Installation#Configuring_and_installing_the_Linux_kernel

""" However, sometimes it's nice to be able to update the kernel all by itself, or leave a kernel update out of your regular whole system update. To do this, we will create a new package set called "kernel". """

This means two things:

1) We can update the kernel without updating the rest of the system by issuing `emerge -auDN --with-bdeps=y @kernel`. It works.

2) We can update the whole system without updating the kernel by issuing `emerge -auDN --with-bdeps=y @world`. This doesn't work!

Here is the output of `emerge -tpvuDN --with-bdeps=y @world`: https://bpaste.net/show/221b8e102634

As you can see, portage wants to update sys-kernel/vanilla-sources, though not in world.