As I know, it is not possible to determine each slot of an Azure web app to a different IP address.
Deployment slots are actually live apps with their own hostnames which is different from Azure web app hostnames. But the slots have the same IP address with Azure web app in Azure. You can use Ping utility to verify it, Ping timeout can say nothing only we can see the host from the web app or each slot in azure.
Here are the screenshots of two slots- staging and full in Azure web app service.

Actually, the IP address 13.90.143.69 is also the IP address of the Azure web app service. If you want dev-slot to the dev-vnet and prod-slot to the prod-vnet. I think it should be dev-slot linked to Azure web app service1 and prod-slot linked to Azure web service2. The service1 has a different IP address with service2.
Hope this makes sense.