defined('VENDORPATH') or define('VENDORPATH', realpath(COREPATH.'..'.DS.'vendor').DS);
So there should be no reason to change this if you have moved the core to another location. All our production installs run of a single shared core installation without problems. Note that 'vendor' is considered a part of the core to, Fuel is designed to have them installed together.
You get that error if
if ( ! is_file(VENDORPATH.'autoload.php'))
is true, so you need to check if you have moved the vendor path too.
If you don't want to move that, you need to define the VENDORPATH in your index.php too, that is not a problem. It is not there by default because nobody ever changes it, so it would only introduce clutter. But you can define it there without problems.