This problem does happen more often than it should. You have two main ways to get it looked into. Either ring 150 from a VM phone or 0345 454 1111 from any phone -> Option 1 -> Confirm your details when prompted -> It'll put you through to the menu, from there choose Option 4 -> Option 1. Ask them to fill in an exception form regarding the unreceived voucher. This will get the situation looked into.
Alternatively wait about three days for a Forum Team member to respond to your topic. A member of said team will look into the situation and appraise you of any updates regarding it.
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.