Many projects have their own policy regarding the code style, so it can happen to you that while working on one of them it is required to use white spaces as indentation instead of commonly used tabs. To activate this feature within the emacs editor only when working with C or Vala source code you can place this in your ~/.emacs file:
(add-hook 'c-mode-hook (lambda () (setq indent-tabs-mode nil))) (add-hook 'vala-mode-hook (lambda () (setq indent-tabs-mode nil)))
A small note: If it should be applicable for a specific project one can combine the above feature with Directory Variables.
Additional references: spaces instead of tabs