How to override vendor directory location

Hi,
I have found this solution extremely efficient when working on Vagrant/Windows.
Maybe others will find it interesting too.

I have tested it in Symfony 2.3 only, so I am not sure if it applies to all other versions.

| Q             | A
| ------------- | ---
| Doc fix?      | [no]
| New docs?     | [no]
| Applies to    | [Symfony 2.3]
| Fixed tickets | [-]

This should be "Override the `vendor` Directory" (according to our title case standard, all words should be capitialized except from closed class words)