My point was that the Samsung number may have changed and Vodafone may not have been aware of that at the time. I also wasn't sure from what you said what time of day you were calling. Many helplines are 24 hours these days, so it wouldn't be unreasonable to think that this one MIGHT be.
As I understand it, when you bought the device, it did what you wanted and it's the update that's caused the problem? If that's the case, then you were correctly sold the device and your case would be with Samsung as it's their upgrade that's caused the problem*. It's strange that they've removed a feature that's a standard component of Android on tablets, but they change a lot of the code - something Google don't like at all, as a matter of fact - and it may be that this has caused security concerns. Either that, or the Google multi-user option isn't as secure as we thought it was and Samsung are ahead of the curve. Clearly, sellers can't predict what may come in future software updates and how these may affect the usability of the device.
You are right, though, that the sales literature is showing the function as being avaiable. It's possible that the devices Vodafone have are still on the version of the OS that includes multiple users, which makes it a bit difficult for them - they can't update their stock and still sell it as new.
Your obvious argument would be that the device isn't as it was sold to you and isn't fit for purpose. The problem with that is that the counter argument is that it was as-described when you bought it and that it's the upgrade, which you applied, that's caused the problem. Release notes do exist, but aren't easy to find and very few people read them - mostly the changes are minor and often cosmetic. I've never seen them put out with an update as a read-me. In this case, that would have been helpful, but I still think most people would have missed it - you are, after all, not expecting a major feature to be removed - though it can happen.
* If the upgrade has been approved by Vodafone, the position potentially changes.
Edited to add: the other possibility is that Google have objected to Samsung marking their device as multi-user, applying it's somehow different to other Android tablets, when it isn't. They've either removed the feature in a fit of pique or the standard Google one isn't compatible with their re-write of Android. In that case, I'd expect it to come back in a later update.
