If there's any evidence of damage, Vodafone aren't allowed by any of the manufacturers to carry out a warranty repair, so you'll need to go direct to Sony on this one. It's possible the phone had a knock some time previously and that the stress has only come out in a crack now, but it's worth seeing what they say. If they want to charge, it might be worth considering a case, which will support the structure of the phone and may be enough to avoid a repair altogether. If it doesn't work, you can always get it repaired later.
Other possibilities would include a 3rd party repair, which may be cheaper but will almost certainly invalidate the warranty - whether that matters depends on how old the phone is - and therefore how soon you can upgrade. Or you could look at a stick-on glass screen protector and put that on the back to stabilise the structure. @BandOfBrothers: am I right in thinking you've used these (albeit not for that purpose)?
