発生したエラー
$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'base' could not be found. Attempting to find and install...
default: Box Provider: virtualbox
default: Box Version: >= 0
==> default: Box file was not detected as metadata. Adding it directly...
==> default: Adding box 'base' (v0) for provider: virtualbox
default: Downloading: base
An error occurred while downloading the remote file. The error
message, if any, is reproduced below. Please fix this error and try
again.
Couldn't open file /Users/***/vagrant/base
どうやら vagrant init の時に box を指定していなかったらしい．
対処法
一旦 Vagrantfile を削除してから作成しなおす．
$ rm Vagrantfile
$ vagrant init CentOS7
$ vagrant up
これで無事仮想マシンを起動できました．