Enabling options based on filetype in vim. Picked from here 

:au BufRead    *.c     so ~/.vim/c.vim
:au BufNewFile *.c     so ~/.vim/c.vim
:au BufRead    *.py    so ~/.vim/py.vim
:au BufNewFile *.py    so ~/.vim/py.vim

Then put this in ~/.vim/c.vim:

set shiftwidth=8 

And this in ~/.vim/py.vim:

set shiftwidth=8 

That'll cause vim to source the specified file when the corresponding file type is read (or when a new file of the corresponding type is created).