VPC with mixed instance tenancy

main.tf

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

resource "aws_vpc" "main" {
  cidr_block = "10.1.0.0/16"
  instance_tenancy = "default"

  tags {
    Name = "test_please_delete_me"
    environment_name = "test"
  }
}

resource "aws_subnet" "public" {
  vpc_id = "${aws_vpc.main.id}"
  cidr_block = "10.1.1.0/20"
  availability_zone = "us-east-1a"
  map_public_ip_on_launch = true

  tags {
    Name = "test_please_delete_me"
    environment_name = "test"
  }
}

resource "aws_instance" "nat" {
  # Ubuntu 14.04 ami
  ami = "ami-d05e75b8"
  instance_type = "m3.medium"
  subnet_id = "${aws_subnet.public.id}"
  tenancy = "default"

  tags {
    Name = "test_nat_please_delete_me"
    environment_name = "test"
  }
}

resource "aws_instance" "test" {
  # Ubuntu 14.04 ami
  ami = "ami-d05e75b8"
  instance_type = "m3.medium"
  subnet_id = "${aws_subnet.public.id}"
  tenancy = "dedicated"

  tags {
    Name = "test_instance_please_delete_me"
    environment_name = "test"
  }
}

output.txt

aws_vpc.main: Creating...
  cidr_block:                "" => "10.1.0.0/16"
  default_network_acl_id:    "" => "<computed>"
  default_security_group_id: "" => "<computed>"
  dhcp_options_id:           "" => "<computed>"
  enable_dns_hostnames:      "" => "<computed>"
  enable_dns_support:        "" => "<computed>"
  instance_tenancy:          "" => "default"
  main_route_table_id:       "" => "<computed>"
  tags.#:                    "" => "2"
  tags.Name:                 "" => "test_please_delete_me"
  tags.environment_name:     "" => "test"
aws_vpc.main: Creation complete
aws_subnet.public: Creating...
  availability_zone:       "" => "us-east-1a"
  cidr_block:              "" => "10.1.1.0/20"
  map_public_ip_on_launch: "" => "1"
  tags.#:                  "" => "2"
  tags.Name:               "" => "test_please_delete_me"
  tags.environment_name:   "" => "test"
  vpc_id:                  "" => "vpc-93a827f7"
aws_subnet.public: Creation complete
aws_instance.nat: Creating...
  ami:                      "" => "ami-d05e75b8"
  availability_zone:        "" => "<computed>"
  ebs_block_device.#:       "" => "<computed>"
  ephemeral_block_device.#: "" => "<computed>"
  instance_type:            "" => "m3.medium"
  key_name:                 "" => "<computed>"
  placement_group:          "" => "<computed>"
  private_dns:              "" => "<computed>"
  private_ip:               "" => "<computed>"
  public_dns:               "" => "<computed>"
  public_ip:                "" => "<computed>"
  root_block_device.#:      "" => "<computed>"
  security_groups.#:        "" => "<computed>"
  source_dest_check:        "" => "1"
  subnet_id:                "" => "subnet-cc6993ba"
  tags.#:                   "" => "2"
  tags.Name:                "" => "test_nat_please_delete_me"
  tags.environment_name:    "" => "test"
  tenancy:                  "" => "default"
  vpc_security_group_ids.#: "" => "<computed>"
aws_instance.test: Creating...
  ami:                      "" => "ami-d05e75b8"
  availability_zone:        "" => "<computed>"
  ebs_block_device.#:       "" => "<computed>"
  ephemeral_block_device.#: "" => "<computed>"
  instance_type:            "" => "m3.medium"
  key_name:                 "" => "<computed>"
  placement_group:          "" => "<computed>"
  private_dns:              "" => "<computed>"
  private_ip:               "" => "<computed>"
  public_dns:               "" => "<computed>"
  public_ip:                "" => "<computed>"
  root_block_device.#:      "" => "<computed>"
  security_groups.#:        "" => "<computed>"
  source_dest_check:        "" => "1"
  subnet_id:                "" => "subnet-cc6993ba"
  tags.#:                   "" => "2"
  tags.Name:                "" => "test_instance_please_delete_me"
  tags.environment_name:    "" => "test"
  tenancy:                  "" => "dedicated"
  vpc_security_group_ids.#: "" => "<computed>"
aws_instance.test: Creation complete
aws_instance.nat: Creation complete

Apply complete! Resources: 4 added, 0 changed, 0 destroyed.

Note that the VPC has instance_tenancy set to default and the first aws_instance resource has tenancy set to default while the second aws_instance resource has tenancy set to dedicated.