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