How to fix Vagrant machine index has become corrupt
The machine index which stores all required information about
running Vagrant environments has become corrupt. This is usually
caused by external tampering of the Vagrant data folder.
Vagrant cannot manage any Vagrant environments if the index is
corrupt. Please attempt to manually correct it. If you are unable
to manually correct it, then remove the data file at the path below.
This will leave all existing Vagrant environments "orphaned" and
they'll have to be destroyed manually.
We are very lucky because removing both the index and index.lock files resolves the problem!
On stackoverflow: http://stackoverflow.com/questions/24911021/vagrant-corrupted-index-file-c-users-username-vagrant-d-data-machine-index-ind