How do I set up a default syntax for files that have no extension in vim?
If I remeber it right you can put a file named syntax.vim inside your ~/.vim/syntax folder. This file is used as default syntax highlight source. If your .vim folder does not exist, you have to create it:
mkdir ~/.vim
mkdir ~/.vim/syntax
touch ~/.vim/syntax/syntax.vim
Now you can add your default syntax to the syntax.vim file. For further documentation you can look at the vim sourceforge page.
Hope this helps.