To add multiple contacts to a vendor, go to Manage Vendors under the Orders tab in the navigation bar. Select a vendor - at the bottom of the Vendor Details modal that pops up, you can add or delete contacts. 

When reviewing your orders on the Shopping Cart page, you can choose which contacts to send your orders to by selecting or deselecting the Email Order checkbox next to their name.

In cases such as a contact being out of office or their email bouncing, you can send the order to another contact by clicking on the specific order on the Order History page, then selecting Send order to.... Either select the vendor contacts you want to send the order to or enter in an email address. Add multiple addresses by clicking the +  button.