With all VM offers 28 days must pass from installation, then they'd be emailed to you (to the email address you used when signing up to VM) by Eden Red I believe, who send the vouchers out on VM's behalf.
If this has passed, please check your emails spam folder, just in case the voucher email is waiting for your attention there. Past the 28 days or so, you can call into 150 / 0345 454 1111, go into the main menu then option 4 -> option 1. Ask them to fill out an exception form on your behalf. In 48 hours it will have been looked into and things will move on from there. A
Forum Team member here can also get an exception form filled in for you (up to three days response from them.) Alternately, call this number 0203 695 3398 - but I've no idea of its staffing hours.