On the Income By Customer Summary report, how do we tie in the expenses (in our case the vendor costs) so it will show on the report?

Turn-On "Track expenses and items by customer" in Account and Settings/Expenses/Bills and expenses. Then be sure the add the Customer to each vendor expense transaction.

How do I access the tracking after I have applied the expense to the customer?

We can pull up the Expenses by Vendor Summary report, abrowning206.

This will show us the total expense amount that are linked to your customers. 

1. Go to Reports tab. Search and select Expenses by Vendor Summary.
2. On the Display columns by drop-down, select Customers.
3. Click Run report.

You'll see that there are added columns in the report to show the total amount per customers.

I have articles here to learn more about reports in QuickBooks Online:

• Reports included in your QuickBooks Online subscription

• Customize your reports in QuickBooks Online

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