Before configuring your reports, the "Custom Fields" feature in QuickBooks is one that you have to set up before it will start working. To do that, go to "Account & Settings" --> "Sales" --> "Sales form content" and click on "Custom fields". It should look like this:
Add a field named "PrimaryProvider" (no space between the words) and set it to be "Internal". Once you've saved that you should start getting this data stored in QuickBooks. (There isn't any other setup to be done in VetBadger, it's just this setting in QuickBooks.) Unfortunately it won't have any historical data before today, but all invoices created from now on will include that data.
A common way to determine sales by provider via QuickBooks is to select the "Sales by Product/Service Detail" report, and then click the "Customize" button (in the upper right). That will open an options panel, and one of the lower options will be "Filter". If you open that section you should see "Primary Provider" as one of the options that you can filter for:
Once you've selected that checkbox, type in the name of the doctor / provider that you're reporting for, and then when you've run the report you can choose to "Save Customization" which will store that report to be run whenever you need it.