Using the shared folder /vagrant will provide really low performance.
If you want decent performance, you will need to do the following:
# create an alias to synchronize the code from the shared folder to a local folder
alias sin="rsync -a --delete /vagrant ~"
# actually synchronize (can take a minute for the first run)
sin
# switch to the local folder
cd ~/vagrant
# standard procedure to update, migrate and launch the application
bundle install
bundle exec rake db:migrate
bundle exec rails s
Enjoy 