outputs.tf

# inside 'common' folder
output "vpc-id" {
  value = "${alicloud_vpc.default.id}"
}

remote-state.tf

# fetch data from common state - https://www.terraform.io/docs/providers/terraform/d/remote_state.html
data "terraform_remote_state" "common" {
  backend = "local"

  config = {
    path = "../common/terraform.tfstate"
  }
}

security-groups.tf

resource "alicloud_security_group" "db" {
  name   = "${var.db-security-group-name}"
  vpc_id = "${data.terraform_remote_state.common.vpc-id}"
}