OpenUDID cannot be shared between Applications on iOS 7+ and now can change. It's still usable but no longer recommended.
The vendorIdentifier hasn't changed, but it's not recommended for Enterprise Apps because it can change when updating the App.
The new advertisingIdentifier API is the way to go for new Apps. It's the same for all Apps and should not change frequently, but the user can reset it in the settings page.
Edit: Website udid.io obtains the real Device UDID instead of the obfuscated one if you need it for anything outside your App. If you need the UDID from within the App, see my answer above.
Well, I know OpenUDID. The problem is though that our (legacy) backend is returning data based on UDIDs. I was looking for a solution which doesn't involve refactoring server side applications. People just had to copy & paste their UDID from iTunes and they were ready to go.
Sorry to hear that, but on iOS 7+ there's no way to access the UDID shown in iTunes from code anymore.