You can delete multiple Vendors at a time. In the Vendor Explorer, select any Vendor(s) you want to delete. You can select multiple Vendors in the grid by holding down Shift (to select a series) or Ctrl (to select outside of a series).
If a Vendor is associated with Project, Task, Service Order, or Service Plan then the Vendor cannot be deleted and you will see a message similar to the one below prompting you to mark the Vendor as "Inactive".