interpolate-provider.tf

provider "aws" {
  region = "us-east-1"
}

provider "aws" {
  alias = "us-east-1"
  region = "us-east-1"
}

provider "aws" {
  alias = "eu-west-1"
  region = "eu-west-1"
}

provider "aws" {
  alias = "us-west-1"
  region = "us-west-1"
}

variable "regions" {
  default = "us-east-1,eu-west-1,us-west-1"
}

resource "aws_vpc" "vpc" {
  count = 3
  provider = "aws.${element(split(",",var.regions), count.index)}"
  cidr_block = "10.1.${count.index}.0/24"
}

This seems to compile without errors:

   variable "count" {
     default = 1
   }

   variable "regions" {
     default = {
        "0" = "us-east-1"
        "1" = "eu-west-2"
        "2" = "us-west-1"
   }

   provider "aws" {
     alias = "0"
     region = "us-east-1"
   }

   provider "aws" {
     alias = "1"
     region = "eu-west-1"
   }

   provider "aws" {
     alias = "2"
     region = "us-west-1" 
   } 

   resource "aws_vpc" "vpc" {
     count = 3
     provider = "aws.aws_${count.index}"
     cidr_block = "10.1.${count.index}.0/24"
   }