@Andrew Eickemeyer glad to see you were able to resolve your issue. Thanks for posting your solution so that others experiencing the same thing can easily reference this.
Since the Microsoft Q&A community has a policy that the question author cannot accept their own answer, they can only accept answers by others, I'll repost your solution in case you'd like to Accept the answer.
Issue Summary:
The issue was related to deleting an Azure Virtual Network due to a service association link that was preventing the deletion of a subnet. This, in turn, was blocking the deletion of an Azure Function App with VNet integration enabled. The error message received was:
"Failed to delete subnet 'subnet-2'. Error: Subnet subnet-2 is in use by rg-aeickemeyer-001/providers/Microsoft.Network/virtualNetworks/vnet-aeickemeyer-001/subnets/subnet-2/serviceAssociationLinks/legionservicelink'>vnet-aeickemeyer-001/subnet-2/legionservicelink and cannot be deleted."
Resolution:
The issue was resolved by disconnecting the VNet integration. you navigated to the networking blade for the Function App and found an extra step under the outbound configurations. There was a hyperlink with the name of the subnet that led to the page with the option to disconnect the VNet integration. Once this was done, the subnet and the associated resources could be deleted successfully.