Its evident there is no parameter for launch config or ASG to define a subnet and consequently a VPC.
I know it's possible and I've seen code that must work (since some are using it), but every time I try I get:
resource "aws_autoscaling_group" "[...]" {
name = [...]
launch_configuration = [...]
max_size = [...]
min_size = [...]
vpc_zone_identifier = ["${split(",",var.private_subnet_ids)}"]
}vpc_zone_identifier = ["${element(split(",", terraform_remote_state.tfremote.output.public_subnets), count.index)}"]