In theory you just have to pay pro rata but VM are famous for charging you another month and then making it awkward to get your money back. Various discussions about it on boards.ie.
To the OP: yes, you are on a month-to-month arrangement, but watch out for any future letters in the post that lock you into a contract by default unless you opt out. Also, even if you decide it's right for you don't acquiesce to any new contract unless you get some sort of discount. Having you on contract is valuable to VM but they will try to play games.
They recently told me I'd be entering a new 12 month contract just by cancelling my TV subscription! After telling them to get knotted I got my broadband speed doubled and a 50% price discount for a period which, in fairness, was no more than they were offering new customers so I wasn't exactly being outrageous.