In your Vim you may have exandtab set which will cause your tabs to be expanded into spaces. See :help 'expandtab'. You can change this in your .vimrc by saying set noexpandtab. Files you have now that need to fixed can be converted by running the :retab command. See :help retab.
You can also see tabs and spaces in your file by enabling setting list, e.g., :set list. This makes whitespace visible and can be controlled by changing the listchars setting. See :help 'list' and :help 'listchars'.