In yet another case, I had to delete the database in RDS first and also replication server and endpoints in DMS. Only after that, i was able to delete the subnets and VPC as they kept on referring to Network Interfaces which were in use. Earlier, i wasn't able to detach or delete neither network interfaces nor internet gateway. They got delete on their own when I deleted the resources+subnets+VPC. Bottomline: Delete the actual resources which may be associated with VPC/subnets/network interfaces/internet gateway first. Otherwise even force delete command won't likely work.