NFS mounts have been known to hang when running vagrant up on Ubuntu. This is commonly caused by issues with using the udp protocol for the NFS mount. In version 1.4.0+ of Vagrant, a new nfs_udp option was added. I was able to add nfs_udp: false to my config.vm.synced_folder options to fix the issue.

config.vm.synced_folder "repositories", "/home/vagrant/repositories", type: "nfs", nfs_udp: false