For the purpose of using classic link, you will need to enable minimum one virtual private cloud on your account for classic link. After doing this, you can associate a security group from that VPC to the EC2 classic instance that you would prefer. This will make sure that your EC2 classic instance is linked to VPC. It will become a member of the chosen security group in the VPC. It should be remembered that you cannot connect your EC2 classic instance to more than one virtual private cloud at the same time.