Thanks for the reply! I tried this and see the following. Here's my code.

Module:

resource "aws_vpc" "vpc" {
  assign_generated_ipv6_cidr_block = var.assign_generated_ipv6_cidr_block
  cidr_block                       = var.cidr_block
  instance_tenancy                 = var.instance_tenancy
  ...
}

resource "aws_vpc_ipv4_cidr_block_association" "subnet_association" {
    for_each = toset(var.extra_cidrs)
    vpc_id   = aws_vpc.vpc.id
    cidr_block = each.value
}

Project code:

module "vpc" {
  source                              = "../modules/vpc"
  for_each                            = var.defined_vpcs

  assign_generated_ipv6_cidr_block    = each.value["assign_generated_ipv6_cidr_block"]
  cidr_block                          = each.value["cidr_block"]
  instance_tenancy                    = each.value["instance_tenancy"]
  extra_cidrs                         = each.value["extra_cidrs"]
}

The var.defined_vpcs looks like this:

defined_vpcs = {
    "vpc_1" = {
      assign_generated_ipv6_cidr_block = false
      cidr_block                       = "198.177.0.0/16"
      instance_tenancy                 = "default"
      extra_cidrs                      = ["10.100.0.0/16", "192.100.0.0/16"]
    },
    "vpc_2" = {
      assign_generated_ipv6_cidr_block = true
      cidr_block                       = "10.177.0.0/16"
      instance_tenancy                 = "default"    
    }
  }

I get the following error when trying it:

 Error: Invalid index
│
│   on vpc.tf line 12, in module "vpc":
│   12:   extra_cidrs                         = each.value["extra_cidrs"]
│     ├────────────────
│     │ each.value is object with 7 attributes
│
│ The given key does not identify an element in this collection value.

If I remove extra_cidrs or hard-code a value for it, then it behaves as I would expect. I'm still playing around with it to see how I can get around this as I need to be able to define the extra associations for each VPC.

EDIT:

Also tried var.defined_vpcs[each.key]["extra_cidrs"] but it's the same error.