Tabs and spaces in vim

Question

How do I prevent vim from replacing spaces with tabs when autoindent is on?

An example: if I have two tabs and 7 spaces in the beginning of the line, and tabstop=3

Answer 1

It is perhaps a good idea not to use tabs at all.

:set expandtab

If you want to replace all the tabs in your file to 3 spaces (which will look pretty similar to tabstop=3):

:%s/^I/   /

(where ^I is the TAB character)

From the VIM online help:

'tabstop' 'ts'      number  (default 8)
        local to buffer
Number of spaces that a  in the file counts for.  Also see
|:retab| command, and 'softtabstop' option.

Note: Setting 'tabstop' to any other value than 8 can make your file
appear wrong in many places (e.g., when printing it).

There are four main ways to use tabs in Vim:
1. Always keep 'tabstop' at 8, set 'softtabstop' and 'shiftwidth' to 4
   (or 3 or whatever you prefer) and use 'noexpandtab'.  Then Vim
   will use a mix of tabs and spaces, but typing  and  will
   behave like a tab appears every 4 (or 3) characters.
2. Set 'tabstop' and 'shiftwidth' to whatever you prefer and use
   'expandtab'.  This way you will always insert spaces.  The
   formatting will never be messed up when 'tabstop' is changed.
3. Set 'tabstop' and 'shiftwidth' to whatever you prefer and use a
   |modeline| to set these values when editing the file again.  Only
   works when using Vim to edit the file.
4. Always set 'tabstop' and 'shiftwidth' to the same value, and
   'noexpandtab'.  This should then work (for initial indents only)
   for any tabstop setting that people use.  It might be nice to have
   tabs after the first non-blank inserted as spaces if you do this
   though.  Otherwise aligned comments will be wrong when 'tabstop' is
   changed.