How to enforce backup of an .ipa from iPad to iTunes/Mac?

Question

Usually, all the .ipas are backup up in the ~/Music/iTunes/Mobile Applications folder. I accidentally deleted an .ipa in the folder. Now, after syncing, the app is on iPad but the .ipa is not backup in the Mobile Applications folder. How can I enforce the sync to backup all .ipas to the Mac again?

My iPad is not jailbreak. I want to restore the VLC app (.ipa deleted from Mac) from iPad and install on another new iPhone ...