As far as the bank and the law are concerned you are not responsible for paying half each - you are both responsible for the whole amount.
If the debt isn't paid and gets as far a County Court Judgement then both of you will be pursued. If one has disappeared or has no income, the whole lot could be taken from the other.
You are referring to an Attachment of Earnings Order, regarding taking the money directly from his wages. This can be arranged after a CCJ is issued if payment isn't forthcoming.
However, it won't happen if you keep paying the debt, and the Attachment could just as likely be on your earnings.
They certainly wouldn't chase half from him and half from you.
If you do sign an agreement with your ex it will be meaningless - the banks won't be interested in anything signed between the two of you.
If you stop payments altogether you will get a CCJ against you.
Tell Vodafone where he is - revenge is sweet.