Contact lenses paid for by NHS voucher?

You will have to pay. The NHS will only issue a new voucher under very strict conditions, such as your next test being due, a change in prescription within recall (i.e. before your next sight test is due), if you're under sixteen and have lost/broken your glasses or if you suffer from a disability which means that you have lost/broken your glasses, but it's not your fault. NHS optical vouchers aren't really designed for disposable contact lenses, so you don't usually get normally get a great deal using the voucher that way.