You can import Vendors from QuickBooks. QuickBooks should be open prior to importing Vendors. This is not required but speed will be improved.
There are filters in the ribbon for showing All, New, or Matched Clients. The New filter shows only Clients who don't already exist in SI. The Matched filter will shows only Clients that already exist in SI, use this for updating information for existing Clients. There are column filters and a search field as well.