IMHO the correct way to install bleeding edge plugins to Vagrant is to use the standard Vagrant installer.
These instructions should work for Vagrant 1.1.0 - 1.4.x (but probably not for Vagrant 1.5+),
git clone https://github.com/berkshelf/berkshelf.git
cd berkshelf
gem build berkshelf.gemspec
# Install directly to Vagrant's gem storage, as this is not a plugin
gem install berkshelf-*.gem --conservative --no-ri --no-rdoc --install-dir $HOME/.vagrant.d/gems
cd ..
git clone https://github.com/berkshelf/vagrant-berkshelf.git
cd vagrant-berkshelf
gem build vagrant-berkshelf.gemspec
# Normal install with Vagrant
vagrant plugin install vagrant-berkshelf-*.gem
In above commands it's possible to also use Vagrant's embedded gem:
- Linux:
/opt/vagrant/embedded/bin/gem - OS X:
/Applications/Vagrant/embedded/bin/gem - Windows:
C:\HashiCorp\Vagrant\embedded\bin\gem