Custom VPC is not defined, default VPC will be used. This may lead to unauthorized network related activities.
New Redshift clusters are created with a default VPC configured. To configure a new VPC, or move the Redshift cluster to another existing VPC, see the AWS documentation.
In Terraform -
References:
https://docs.aws.amazon.com/redshift/latest/mgmt/managing-cluster-cross-vpc.html
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/redshift_cluster#vpc_security_group_ids