Disassociates a CIDR block from a VPC. To disassociate the CIDR block, you must specify its association ID. You can get the association ID by using DescribeVpcs. You must detach or delete all gateways and resources that are associated with the CIDR block before you can disassociate it.
You cannot disassociate the CIDR block with which you originally created the VPC (the primary CIDR block).