If I understand your question correctly, the following is added to your .vimrcshoud work
autocmd BufRead,BufNewFile *.waf set filetype=python
If this is a specific file name, for example wscript, this also works:
autocmd BufRead,BufNewFile wscript set filetype=python
If you cannot rely on the extension or file name, you can add the model at the top or bottom of your file.
# vim: set filetype=python :
. :help modeline.
, , , .