To redeem these vouchers, you need to call Virgin Atlantic. In both the companion and the upgrade, it has to be on a Virgin flight, so if you are in NZ you'd need to make your own way to Sydney, Asia or Los Angeles to use them.
For the companion voucher, you need to pay for a very expensive flexible ticket, and also have award availability for that flight in the class you want. Go here (you probably have to log on to your Flying Club account) to look at the expensive fare classes you would need to pay for in order to use the companion voucher.
http://www.virgin-atlantic.com/en/gb...ghts/index.jsp
You will 95% of the time for holiday travel find it much cheaper to just buy two tickets so, unless you really need a flexible ticket or are booking a few days in advance, don't bother with the companion voucher. (it's only really much good for bringing somebody along on a last minute business trip)
For the upgrade voucher, it's quite simple. If you see award availability in Premium Economy, you ring up Virgin Atlantic and ask to book it using your upgrade certificate, and they will charge you the miles for an economy redemption.
Not sure how long you've been in NZ but the UK MBNA vouchers expire after a year...
Hope that helps...