There are various ways that you can add Vendors to your SI 2015 database.
You can import multiple Vendors at a time from Outlook. Click here for details.
You can import multiple vendors at a time from QuickBooks. Click here for details.
You can add a Vendor "from scratch" via the New button. Click here for details.
Import Vendor(s) from Outlook
Import Vendor(s) from QuickBooks
New Button
Deleting Vendors
Editing Vendor