Add EC2 VPC facts to Facter 1.7, drop this file into `/etc/facter/facts.d/`

ec2_vpc.rb

require 'facter/ec2'

# We might be in a VPC, which fails the normal EC2 MAC address checks
if !Facter.value('ec2_instance_id') && Facter.value('virtual') =~ /xen/ && Facter::Util::EC2.can_connect?(0.2)
  Facter.debug "This is an EC2 VPC instance"
  metadata
  userdata

  # vpc-id is in a newer metadata api rev than the usual EC2 facts
  Facter.add(:ec2_vpc_id) do
    setcode do
      mac = open("http://169.254.169.254/2011-01-01/meta-data/mac").read.chomp
      open("http://169.254.169.254/2011-01-01/meta-data/network/interfaces/macs/#{mac}/vpc-id").read.chomp
    end
  end
else
  Facter.debug "Still not an EC2 host"
end

When, however, I dropped this into /usr/lib/ruby/site-ruby/1.8/facter, it worked fine.