Dear Philipp,
Unfortunately, WooCommerce does not have a proper developer program, so I don't always have access to the latest version of WC Product Vendors. The latest version that I was able to test was WC Product Vendors 1.2.3 (just tested a minute ago, and the shipping was properly including the length of the vendors list). Apparently WooThemes must have changed something in the plugin in an incompatible way, so our plugin broke. As I said, I don't have a copy of the latest version, so I can't debug or fix the problem...
Shall I refund your order? Alternatively, if you can provide me with a copy of WooCommerce Product Vendors, I can check whether I can quickly spot a fix...
Best regards,
Reinhold