vagrant において、明示的に VirtualBox の機能(vboxsf)を使いフォルダを同期するには、`config.vm.synced_folder ".", "/vagrant", type: "virtualbox"` のように Vagrantfile に設定するとよい ref: http://qiita.com/toby_net/items/6eb74471871ab9fba087

file0.txt

# インストール
> vagrant plugin install vagrant-vbguest

# 確認
> vagrant plugin list
:
vagrant-vbguest (0.11.0)
:

file1.diff

diff --git a/Vagrantfile b/Vagrantfile
index xxxxxx..xxxxxx 100644
--- a/Vagrantfile
+++ b/Vagrantfile
@@ -38,6 +38,7 @@ Vagrant.configure(2) do |config|
   # the path on the guest to mount the folder. And the optional third
   # argument is a set of non-required options.
   # config.vm.synced_folder "../data", "/vagrant_data"
+  config.vm.synced_folder ".", "/vagrant", type: "virtualbox"

   # Provider-specific configuration so you can fine-tune various
   # backing providers for Vagrant. These expose provider-specific options.

file2.txt

$ mount | grep vagrant
none on /vagrant type vboxsf (rw,nodev,relatime)
$ echo "foo\nbar" >> /vagrant/test.txt

file3.txt

> cat test.txt
foo
bar

file4.rb

synced_folder("rsync", 5) do
  require_relative "synced_folder"
  SyncedFolder
end

file5.rb

def self.synced_folder(name, priority=10, &block)
  components.synced_folders.register(name.to_sym) do
    [block.call, priority]
  end

  nil
end 

file6.rb

synced_folder(:virtualbox) do
    require File.expand_path("../synced_folder", __FILE__)
    SyncedFolder
  end