To view vendor report, use Reports button available in the view toolbar (regardless which view is currently opened, this button is always available):
By default, drop-down list under this button contains only one report - Vendor performance. It looks like this:
It shows information about total number of assignments with different statuses, specializations chart, average rating chart, unpaid assignments, unpaid invoices and amounts of unpaid invoices.
To create additional report and allow vendor to use it, please follow recommendations from the Providing access to additional reports for Vendors article.