Override Vagrant configuration settings locally (per-dev)

I would suggest using environment variables to dynamically change the behavior of the Vagrantfile without editing the file itself.

To give a real world example, here's how you could use an Ubuntu base box by default but have an environment variable define an alternative Linux distribution:

if ENV['OPERATINGSYSTEM']
  if ENV['OPERATINGSYSTEM'].downcase == 'redhat'
    os_name = 'centos'
    config.vm.box     = 'centos'
    config.vm.box_url = 'https://dl.dropbox.com/u/7225008/Vagrant/CentOS-6.3-x86_64-minimal.box'
  else
    raise(Exception, "undefined operatingsystem: #{ENV['OPERATINGSYSTEM']}")
  end
else
  os_name = 'precise64'
  config.vm.box     = 'precise64'
  config.vm.box_url = 'http://files.vagrantup.com/precise64.box'
end

This example comes from https://github.com/puppetlabs/puppetlabs-openstack_dev_env

The Vagrantfile is just Ruby, so YAML is another option.

For example, in the Vagrantfile I do this:

# -*- mode: ruby -*-
# vi: set ft=ruby :
require 'yaml'

settings = YAML.load_file 'vagrant.yml'
db_ip_address = settings['db']['ip_address']
api_ip_address = settings['api']['ip_address']

Vagrant.configure("2") do |config|
  config.vm.box = "ffuenf/ubuntu-13.10-server-amd64"
  config.vm.box_url = "https://vagrantcloud.com/ffuenf/ubuntu-13.10-server-amd64/version/4/provider/virtualbox.box"

  config.vm.define "db" do |db|
    db.vm.synced_folder settings['db']['artifacts_dir']['host'], settings['db']['artifacts_dir']['guest']
    db.vm.network "private_network", ip: db_ip_address
    ... other stuff ...
  end

  config.vm.define "api" do |api|
    api.vm.synced_folder settings['api']['artifacts_dir']['host'], settings['api']['artifacts_dir']['guest']
    api.vm.network "private_network", ip: api_ip_address
    api.vm.network "forwarded_port", guest: settings['api']['forwarded_port']['guest'], host: settings['api']['forwarded_port']['host']
  end
end

Then I have a vagrant.yml file (I just made up the name; you can use whatever name you like) for the developer-specific configuration:

db:
  ip_address: 192.168.4.14
  artifacts_dir:
    host: /Users/willie/myapp/db-scripts
    guest: /opt/myapp/db

api:
  ip_address: 192.168.4.15
  forwarded_port:
    host: 9080
    guest: 8080
  artifacts_dir:
    host: /Users/willie/myapp/artifacts
    guest: /opt/myapp/api