Turning on XML syntax highlighting in vim for a non-XML file

The command you want is ":setfiletype". For example:

:setf xml

See ":help :setfiletype".

You can place this in your ~/.vimrc file as well

au BufNewFile, BufRead *.extension_name set filetype=xml

Replace extension_name with the extension you want, reopen vim, and you will see xml syntax highlight.