Just do a SYNC. Although it is theoretically possible for Apple to remove apps from your iPad this has never been done.
If you SYNC, the app will be saved to iTunes, and will be available to you on any iDevices it was designed to run on.
I have VLC on my iDevices, and regularly uninstall it to release the storage that it uses. VLC has not been available for several months now, but I am able to use it on all of my devices, including those purchased since the app was pulled.
The fact that the app shows grayed out in your iTunes is nothing to do with it's non-availability in the app store. Once you have paid for it, it is yours for ever.
There is something else going on here.