I have split my .vimrc into several files and placed them into ~/vimfiles/vimrc.d/.
Currently I source each file in that directory using exact name:
mb14 gave you the best answer. You want something automatically executed? Then use the standard organization: here the plugin/ subdirectory.
Otherwise, :runtime would have been your friend:
:runtime! vimrc.d/*.vim
:source barks when its parameter doesn't exist while :runtime silently source nothing. :source can source only one file while :runtime! can source any number of files.:source takes an absolute pathname, or a pathname relative to the current directory while :runtime takes a pathname relative to the 'runtimepath' option, which shouldn't be a problem as long as you respect vim conventions.