Setting vim options only for files in a certain directory tree?

Central configuration

If it's okay to configure the local exceptions centrally, you can put such autocmds into your ~/.vimrc:

:autocmd BufRead,BufNewFile /path/to/dir/* setlocal ts=4 sw=4

On the other hand, if you want the specific configuration stored with the project (and don't want to embed this in all files via modelines), you have the following two options:

Local config with built-in functionality

If you always start Vim from the project root directory, the built-in

:set exrc

enables the reading of a .vimrc file from the current directory. You can place the :set ts=4 sw=4 commands in there.

Local config through plugin

Otherwise, you need the help of a plugin; there are several on vim.org; I can recommend the localrc plugin, which even allows local filetype-specific configuration.

^{Note that reading configuration from the file system has security implications; you may want to :set secure.}

You can configure vim to read further commands using the source (so) command. Add this to your ~/.vimrc - it searches the current directory and if .vimrc_proj file not found there searches for .vimrc_proj in the parent directory.

if filereadable(".vimrc_proj")
    so .vimrc_proj
else
    if filereadable("../.vimrc_proj")
         so .vimrc_proj
    endif
endif

Then add any custom commands in .vimrc_proj config files to suit your projects.

You can use a plugin for Vim to solve the problem in a more general way by trying to detect the indention.

The plugin of choice for me is DetectIndent. It took some time for me to test all the forks of the plugin to find one that suits my needs. The original one was really close but not quite so I made my own fork.

For debugging it is very helpful to :set verbose=1 and run the plugin again with :DetectIndent