Site moved to new server - Error 500
The main issue is that the require_once CRAFT_VENDOR_PATH.'/autoload.php'; fails.
This is because the DIR constant, used to compute the value of CRAFT_BASE_PATH and CRAFT_VENDOR_PATH is undefined.
Try to define the value of the DIR constant so that CRAFT_VENDOR_PATH points to the vendor directory in your Craft project (it hard to help you further without knowing your directory structure).
Once this is done, as Jamie mentions, you must use composer to install Craft dependencies.
Did you make any changes to the index.php file of is it just like it was in the archive you received from the other dev?