vim search and replace

gistfile1.txt

I have this file containing a lot of this

    authorized_cidr = ""
    concourse_keys = ""
    encryption_key = ""
    instance_ami = ""
    name_prefix = ""
    postgres_database = ""
    postgres_host = ""
    postgres_password = ""
    postgres_port = ""
    postgres_username = ""
    private_subnet_ids = ""
    public_subnet_ids = ""
    
I need to transform into this:

    authorized_cidr = "${var.authorized_cidr}"
    concourse_keys = "${var.concourse_keys}"
    encryption_key = "${var.encryption_key}"
    ... etc

I'm trying vim, like this:

%:s:/\(\s*\)\(\w*\)\(\s*\)=\(\s*\)"":\1\2\3= "\$\{var.\1\}"

but it doesn't work. It reports the error "Pattern not found: /\(\s*\)\(\w*\)\(\s*\)=\(\s*\)"""

%s/\s*(\w*)\s*=\s*"\zs\ze"/${var.\1}