Hello malcolmstevens, welcome on the Vodafone forum! If you cancel your subscription before the end of the contract we will charge the remaining monthly costs. If you prefer to let the contract run to term, this is ofcourse a possibility. Don't forget to cancel your subscription per end of contract though (one month in advance), because if you forget this the contract will continue after the end date.
For you, the cheapest option would be to change the contract from your name to someone else (if someone is willing to take over the contract from you). This way the contract continues on someone elses name, who can use it till the end of contract (and even prolongue if he/she wanted to). Are you by any chance coming back to the Netherlands in the future? You can freeze your contract for 9 months then before it continues.