My goal is simple:
Allow any instance inside a VPC to connect to a specific security group.
The field for allowed sources suggests: "CIDR, IP or Security group".
Is there a way for me to specify a VPC?
The reason beeing that I have multiple instances inside a VPC with different security groups but all need access to the security group I am creating.
Your VPC has a CIDR group range, so just specify the CIDR group that is being used by your VPC and it will allow access from anything in the VPC.