Redshift clusters that are created without subnet details will be created in EC2 classic mode, meaning that they will be outside of a known VPC and running in tennant.
In order to benefit from the additional security features achieved with using an owned VPC, the subnet should be set.
Redshift cluster does not benefit from VPC security if it is deployed in EC2 classic mode
Deploy Redshift cluster into a non default VPC
The following example will fail the aws-redshift-non-default-vpc-deployment check.
The following example will pass the aws-redshift-non-default-vpc-deployment check.